Most of us aren't anywhere near doing this yet, but very experienced choirs face a huge wealth of magnificent propers in polyphonic settings that can be sung as a replacement for the chant. What's wonderful here is the addition into the Mass of these intervals of great color and beautiful during processions and contemplative portions of the Mass, while the people continue to sing the ordinary.
Of course this is impractical for any regular schola in a regular parish. But when it is possible, it is glorious. I'm so blessed to have experienced this for the first time at the Byrd Festival as the masterful Cantores in Ecclesia sang the polyphonic propers written by William Byrd.
I'm so delighted to be able to offer you a sample in the form of the following MP3s recorded on the spot (by an NLMer) during a Pontifical Mass for the Feast of the Assumption of the Blessed Virgin Mary 2008. The recordings come complete with crying babies and slamming kneelers, the usual parish racket amidst splendid beauty, that mix we all truly and seriously love.
Here we go:
Gaudeamus
Propter veritatem
Assumpta est Maria
Optimam partem
I would dearly love to have a complete set of Cantores singing the entire cycle!