And I nodded my head and felt sympathy for you. I also used the I_doubt_its_maintained_anymore and just_barely_works xemail-net to write a live GMail backend for beagle. It does not index the emails as of now, but uses the IMAP search protocol to directly search the emails on the GMail IMAP server. And searching followed by retrieving the headers of the matched emails is really slow; the delay is clearly perceptible. It could be due to xemail but I could not find a better alternative for a .Net IMAP library.
It should not be hard to take the current backend and add the ability to download a batch of emails and index them locally. Google also publishes a nice set of GData .NET API for accessing Google documents, calendars and a lot of other services. A backend for them would at least make our beloved maintainer happy.
A basic GMail query-only backend was on my TODO list for more than year. And now its finally done. Proper GMail indexing and Google service is also on my TODO list. So hope to see it sometime... say by next year.