As far as I know, the only "good" air to hotel conversion is AA->HH. That's one that I consider roughly lossless, based on typical mile/point valuations.

I can't even come up with a double-conversion for DL or US. I'm pretty much stumped on anything that could yield decent value. Do either of those swap through Amtrak? (Amtrak is a decent 'middleman' for double-conversions when it works.) If you're flying a bunch of Skyteam, I think you could do some CO->Amtrak->HH...