At first I simply made the change Carlos suggested, but then the grammatical genes from my mother kicked in and I decided that "polytextual" (adjective) is to be preferred over "polytext" (noun) in the category name. I hope others don't object.
One work that belongs in this category and which I know intimately from performance and from recording it with Zephyrus is the Obrecht Missa Sub tuum presidium. I have been preparing my own edition of this work, but we recently had one posted to ChoralWiki. It is a truly amazing work ranging from 3 parts in the Kyrie, then adding an additional part with each successive movement until there are 7 parts in the Agnus Dei. The title chant is clearly heard in the top voice throughout the work, and five additional Marian chants are found in movements other than the Kyrie.