I got a response from one of the experts on rotax-owner.com.  Here's our exchange:

Here's the EGT/MAP trace I'm referring to in my last post.  Is my sensor measuring the wrong thing?  I don't recall ever seeing anything below ambient.  Ugh.

Well, the engine and plane were perfectly fine on flights this past Sunday and Monday, but then this happened on Tuesday...  Barely starts and even if advancing the throttle it won't get above about 780 RPM.

It looks like it's drawing only from the battery and never from either of the alternators.  That's not sustainable, right?

Thoughts?