The modal theory for chant was used all the time, you used it to know which chant melodies to put together (for the most part a service didn't change mode very much). This is practiced to an even greater extent in the Syrian church, where specific modes go with each period of the ecclesiastical year. In this sort of context everybody who sings knows the mode they're in for the week.

It's less important with polyphonic music, but even quite complex Renaissance polyphony was usually based around a tenor with a definite mode which might be meant to go along with monophonic chants in the same mode.

You might want to look at that Bach chorale site to see if his arrangements give you any ideas for accompaniments. They were pretty complicated.