OP here - Yes (to Horton333), I already have something that will work (400w inverter, plugged into bedroom 12V outlet). As others have said, that is inefficient (converting 12v DC to 120v AC to 24V DC), but it should work. I was concerned about the lack of PSW inverter hurting the CPAP - doesn't sound like that will be an issue.
Another Yes (to 2oldman), I'm reluctant to part with the $85 unnecessarily. I'll try the inverter for a couple days next week; might end up getting that DC converter.
Thanks to all for the feedback,