"The second is like unto the first" as they say. Canon law, as I understand it, actually forbids Catholics to actively participate in leadership roles in the worship and rituals of ecclesial communities not in union with the Holy See. I'm not entirely clear on this, but I believe (and perhaps someone reading this who understands it better can clarify) that it has the potential for punishment by excommunication. On the other hand, I'm sure that for those of us who rely on church music as our sole means of income, an exception can be made.

To understand any text in the Code’s book on penal law, one must first go to canon 18 which indicates that laws which establish a penalty or restrict the free exercise of rights must be interpreted strictly, i.e. in the narrowest possible fashion.

Also, in the document promulgating the Code Pope John Paul made it clear that the Code was to put into canonical form the ecclesiology of the Council and that the Code must be referred to the Council for understanding and interpretation (not the reverse) (Sacrae disciplinae legis, xiv). Just a few paragraph’s later the text highlights the “Church’s commitment to ecumenism” as one of the “elements which characterize the true and genuine image of the Church.” Given that Unitatis redintegratio specifically envisioned some circumstances of common worship (#8), this must also be taken into account.

Within this context, we can look at canon 1365 that speaks of “prohibited participation in sacred rites (vetitae communicationis in sacris). The term “communicatio in sacris” is not defined in the Code, but is understood by the Ecumenical Directory to refer to sharing in the “liturgical life” of a community —and hence might at first glance seem to apply. But we must be cautious. The canon speaks of “prohibited participation” in such rites. The canon does not prohibit all such participation. Thus, to spread the penalty for communicatio in sacris to any and all participation, even leadership (such as music), both interprets the canon broadly, not strictly – a violation of the critical principle for interpreting all penal law – and neglects the adjective.

So what is “prohibited participation”? The Ecumenical Directory, issued in 1993 and replacing an earlier Directory of 1967, has a fairly detailed discussion of this issue. You might want to look at the section from #102-142 to catch the whole picture, but let me pull out a few texts to ease the concern you raised.

In term of sharing in “non-Sacramental Liturgical Worship (which is probably what is at issue unless we are talking about a celebration of “the Lord’s Supper” in a higher liturgical tradition) indicates that in taking part in the liturgical celebrations of other churches and ecclesial communities, Catholics are “encouraged to take part in the psalms, responses, hymns and common actions” and may “read a lesson or preach” if invited. Given the importance of Scripture in most of the reformed traditions, to be able to read and even preach is far more leadership than leading the choir or playing instruments. An ancient principle of canonical interpretation is that “one who can do the greater can do the lesser” unless specifically provided otherwise.

When it comes to participation in sacramental activities of churches not in union with us, distinctions are made between the Eastern Churches and others. I will not reflect on the norms for the east here. For other Christian churches, the Directory states that the key is to remember Eucharistic celebrations are “inseparably linked to full ecclesial communion and its visible expression” (#129), while adding that baptism does create a “real, even if imperfect communion, with the Catholic Church.” (#129). After stating these principles, the Directory pretty much gives norms only for the participation of others in our Catholic rites, not the reverse. In paragraph 132, the Directory does indicate that Catholics without access to their own sacraments “may ask for these sacraments only from a minister in whose church these sacraments are valid” or (interestingly “from one known to be validly ordained according to the Catholic teaching on ordination.” Clearly, then, receiving Eucharist would be a “prohibited participation” in sacred rites. Any sort of participation that would be an expression of severing or undermining the Catholic’s full communion would also be prohibited. Otherwise, there is no clear prohibition in the documents for other kinds of participation.

Going back to the principle of strict interpretation, unless something is clearly prohibited, then the penalty does not apply. (This prescinds from the wisdom of participation, or the possibility of giving scandal).

Finally, for completeness, another principle of interpretation is that when the meaning of a canon is “doubtful or obscure” then “recourse is taken to parallel places” (c. 17). The Code of Canons for the Eastern Churches, while not law for Latin Catholics, is such a parallel place. In fact, because it came out in 1990 and the drafters benefitted from seeing how the Latin Code was received, they often refined texts or made changes that are very helpful. Canon 670.1 of the Eastern Code has no Latin parallel. It states: “For a just cause Catholics can attend the liturgical worship of other Christians and take part in the same, observing those things which, by reason of the degree of communion with the Catholic Church, are established by the eparchial bishop or by a superior authority.” Note that this speaks specifically of liturgical participation, not just other forms of prayer. The gist is that such participation must in no way betray indifferentism, a defection from faith, etc. The second paragraph of this canon allows the use of a Catholic church for non-Catholic Christians in certain circumstances. Given that the Eastern Code clearly expresses a freedom for Catholics to take part in such ceremonies, to argue that Latin Catholics have no such freedom is not tenable.