If you are singing the Communion proper in Latin, then assuredly it is Psallite Domino as in GR. The Missal antiphon Ecce ego vobiscum is a spoken alternative and MUST NOT be sung (in Latin). The reformers, whatever their faults, were very explicit about that. There is no authentic chant for that text in Graduale synopticum.

GIRM 87. In the Dioceses of the United States of America, there are four options for singing at Communion:
(1) the antiphon from the Missal or the antiphon with its Psalm from the Graduale Romanum, as set to music there or in another musical setting; (2) ...

A note in the Graduale (p. 13) may give you the "out" you're looking for: "In omnibus Missis de Tempore eligi potest pro opportunitate, loco cuiusvis cantus diei proprii, alius ex eodem tempore", translated roughly as: "In all masses of the proper of the time, one is allowed to choose a chant from the same season in place of the proper chant of the day." So since "Psallite" is a chant of Ascensiontide, you can legitimately sing it in place of the chant assigned to the day in Year B.

(FYI, an interesting discussion on the Forum on this topic of "seasonal" propers.)

Thanks for the input. I suppose my concern is not so much whether or not the practice is licit (since it most assuredly is), but rather whether or not this would be considered to be poor form. I'm also curious about whether or not others on the forum choose to do this in similar situations.

CGZ the option of Ecce ego vobiscum is not at issue, that was my mistake. The question is whether the antiphon given in GR(1974), Signa eos, should be preferred to Psallite Domino, which is given in GR(1974) for next year and is the only one for Ascension given by LU(1961). Signa eos is a direct quotation from the Gospel reading this year.

That's also just your opinion… That is, it doesn't actually answer the question: is it licit to sing a proper chant that is given in the traditional liturgy and for one or two years in the modern Graduale Romanum, when another one is provided for other years (usually due to the Gospel cycle)? The answer, I think, is "yes," but some people might want more justification than that;

And yes, it's "random," in the sense that there's no rhyme or reason to change the proper: sometimes there are three options, sometimes two, very rarely just one… I'm not sure which is the real problem: forcing the communion to fit the Gospel more regularly, or changing the Gospel cycle in the first place.

a somewhat random communion

As other commenters above have pointed out, the choice of the communion antiphon is not quite random. Actually, for Year B the set of propers with Signa eos is even better matched. There is a correspondence between the Scripture read at Mass and what is sung at communion: it "reminds us of the intimate relationship between the Book of Gospels and the altar, between the Word and the Eucharist" (Boselli, p. 67).

Goffredo Boselli in his The Spiritual Meaning of the Liturgy (2014, pp. 68-69) offers a wonderful mystagogy of the communion antiphon: "Also significant is the fact that the communion antiphon is proclaimed just prior to the distribution of the Eucharist to the faithful. Preceding communion, the gospel verse becomes an invitation. The same word of God that has called the believer into his presence now invites the believer into communion with God, to be nourished at the table of the bread of life and to drink from the chalice of salvation: "Wisdom has mixed her wine, she has also set her table, and proclaimed: 'Come, eat of my bread, and drink of the wine I have mixed'" (see Prov 9:1-5). But the communion antiphon proclaimed before receiving the Eucharist is also an appeal, almost an admonition addressed to the faithful: do not nourish yourselves on the eucharistic body of the Lord if you have not heard, accepted, and obeyed his word. There can be no true manducatio panis without a manducatio verbi."

But, even though I think Signa eos is the better option here, there's nothing wrong to substitute it with Psallite Domino. Go ahead!

By the way, that has nothing to do with using the option of alius cantus congruus (yes, updated language since 2002), but with legitimately using the available options in the Graduale Romanum. Alius cantus congruus comes in play when you substitute the chant from the Graduale Romanum or Graduale simplex with a chant or song from another approved collection.

I suppose my choice of the word "random" was a bit hyperbolic. The liturgical crisis being what it is, I suppose I (perhaps wrongly) tend to view changes like this with overt skepticism.

"Random" assumes a lack of plan. Read Bugnini's books to learn otherwise.

the minor propers are matched to the pericope as a whole rather than each other

That's true for feasts and seasons. Although, I would rather say that they are more matched to the feast or the seasonal day instead of the Gospel pericope. But outside feasts and seasons (i.e. during tempus per annum), the minor propers are generally aligned individually with their liturgical place and function instead of with the readings, with the occasional exception of the communion antiphon.
Liturgies per annum don't have a specific 'theme', but are an amalgam of excerpts from Scripture that together show light on the mystery of salvation (only the first reading and the Gospel are somehow aligned).

Was the tridentine Per signum Crucis just the replacement of a local variant or a 'reform'?

If this was a reform, then certainly not tridentine. Per signum is used by Roman curia at least since 13th century. Perhaps local variation.