A doctor asked me that-- a fact I found disturbing, because shouldn't he know-- so I did some research. And I found out that the machine would spread the virus everywhere with the outgassing (I'm not sure if that's the right term).
There are way too many different brands and types of CPAP machine for anyone to make any sort of sensible comment here. They aren’t all the same. Some of them do have a backup rate that could be used to mandatory mechanical ventilation, but that can’t be readily altered at the bedside in the models I have used. The use-case is radically different to the ICU.