I can confirm that setting up a DavMail in server mode & using it as a caldav gateway to exchange works fine
I've tried it on an f-droid only version of Android without Exchange provider
Only tried caldav but no doubt the other gateways DavMail provides would work too
If it works on android, no reason why it wouldn't work for UBPorts
It'd be nice to have it on-device though, maybe DavMail could be repackaged in some way to provide the required Dav gateways to Exchange
I know it's java based though, so might be too heavy to run in the background on a mobile device
But it's a thought