To be frank, it didn't work for us.

There are a couple of websites selling specialized singing masks that have a structure almost like a bird cage to keep the mask off of your face (there are also silicone inserts that can do this to any mask) which could be of help... but they so muffle the sound and restrict airflow we just had to give up on it. We are a small group of intimates, however, and many of our schola have either had it or were vaccinated (or both) so our risk level is fairly low at this point.

It should also be noted that masks can be reasonably effective at filtering air on the way in, but do comparably little to filter on the way out, as the pressure causes the edges of the mask to flap open even if you can't see it. There are videos floating around that show how much of the air you breathe out of a mask actually shoots out the sides and up toward your eyes on either side of the nose where they tend to only fit loosely.

How can singing with masks be done, either singing chant Propers or any kind of polyphony, safely?

How anyone can breath properly with those on while doing something like singing is beyond me. Reducing your airflow while singing is never a good idea. I'd go without.

Stulte,

If it were my decision....

I know, I know. We live in difficult times.

Reducing your airflow while singing is never a good idea

On the contrary, an efficient and economical airflow is always for the better, and masks have been a valuable feedback for some of my singers. Jogging up the five flights of stairs at the library while masked, as I did yesterday, is a different matter, but I can still distinguish discomfort from 'danger'.

Chris,
No one passed out, all of us stayed together, we do have choir mics and they were used, our tone and pitch were not affected as far as I could tell. Are you familiar with the choral piece Psallite, I think that's the right spelling, we did a cappella for prelude. We had two tenors, one baritone, four sopranos for our small group. As a forty year choir veteran, the choir did quite well (IMHO). Our actual Christmas program is listed on another forum topic, Christmas 2021, I think.

Safe singing is indeed possible with (the right kind of) masks or masks with an insert/bracket/frame.

Christmas With Zephyrus 2020 (the ensemble I sang with from 1992-2004):
https://www.youtube.com/watch?v=tZJjsVCb8h8&t=57s

2021 The Phipps Festival Chorus | We Make Joy Now in this Fest!
https://www.youtube.com/watch?v=Km4ulI_XTFQ

Most of the singers with the Festival Chorus used a mask insert/bracket/frame which made it rather easy to breathe and sing without undue extra effort or inconvenience.

We use the type of mask Chuck is talking about. Got ours on Amazon through a seller called Southeastern Performance Apparel. It has a kind of bracket/frame that keeps the material away from your mouth for easy breathing and good diction. The sound comes through quite well also, but it is fitted nicely to prevent particles from escaping.

We used them this Advent while singing Giffen's harmonization of "Come Thou Long-Expected Jesus"!
;-)

I ended up with a very warm & damp face, every time. The masks gave me a sore throat each time, and I'd then lose my voice.
It's clearly not harmless for everyone.

Corinne, have you discussed this with your doctor?

I've ended up with a very warm and damp face in the loft of a 19c church in Maryland in July at my brother in law's wedding, with my bare feet in a tub of ice. That I didn't lose my voice is anecdotal evidence, of course, and I can easily believe we might be made of different stuffs.

Sponsa,

I'm distinctly NOT trying to normalize it, for what it's worth. I try to avoid normalizing the wearing of masks in unwarranted situations (that is, most of the time).