Might we adapt the standard tonal warm-ups? Take the 1-9-1 scalic warm-up, for example, and sing it in each of the modes?

Michael, there are some wonderful exercises in the back of Ward 4, but they are in number notation. The Neume version is in the updated edition. I'm hoping that someone will just transcribe those and we can post them. They are fantastic.

I use the sight-singing excercises in Ward 4. They are very good both for vocal excerscises and sight singsing. I don't really use numbers for adult schola (I use them for children.). Some finds that numbers help them though. In that case, they just write a number chart and write the corresponding solfeges under the numbers. They can transfer the numbers to solfege pretty easily and get used to it, because the excercises are pretty simple. Eventually they don't need to look at the numbers and sing from the notation directly.
There are many other good excercises of different modes in this book, but many of them are in numbers. It's not too hard to transfer the numbers to solfege, once you write the chart with the excercises. You get used to it. Also, in the same book, there are some good excercises for different kind of neums. For example, Pg.59-60, has good excercises on mora vocis and podatus singing.
Another book I use is Sunol' book, Pg. 7-19. I use them to have them also do the Arsis and thesis gestures. The gestures helps them to feel the motion of the melody, so they don't sing like wearing a 'straight jacket'. They are not in different modes, but you can adapt them to different modes.
These books are written by musicians of Solesmes method. But I think those excercises are very useful whether use strictly follow Solesmes method or not in your singing chants. (In other words, you don't have to count 1,2,1,2,--, if you don't want to.)