I was browsing through "By Flowing Waters" and the Simple English Propers and I made a few chance discoveries. 4 of the Introits and 3 Offertories in ordinary time from BFW match or almost match those found in the SEP. I verified this by checking them against the Graduale Simplex and the Graduale Romanum.

Now, if you follow my reasoning for a moment:

1.) It is permitted that any chant proper may replace the same chant proper within the same liturgical season. Eg. Any introit in Ordinary Time may substitute for any other introit in ordinary time.

2.) There are 9 chant masses in the Graduale Simplex and they are not assigned to any particular sunday, but I imagine that it is expected that a number of them would be sung in rotation throughout the 32 Sundays of Ordinary Time.

It therefore seems logical to me, that any of the SEP chants which match the texts of the GS chants should be used in preference when substitution is taking place.

At a stretch, it could be argued that these chants are in fact ad libitum for ordinary time.

The Introits and Offertories in question are:

Introits: Ordinary Time

Sunday II
Sunday XIV
Sunday XXI
Sunday XXIV

Offertories: Ordinary Time

Sunday II
Sunday VII
Sunday X

So, in Combination with the 7 Ad Libitum Communion Chants, it is possible to put together a collection of Seasonal Chant Propers for Ordinary Time, effectively creating a collection of Ad Libitum Introits and Offertories for the season of Ordinary time.

Now, I am not advocating that this be the ordinary case (no pun intended), but it would seem to be that these chants should be well-known in your parish choir as an emergency back-up.