I had often wondered about this too. I've always speculated that as Christine is close with Steve Winwood and Mason & Winwood obviously didn't get along in their Traffic days...
I'd like to think Christine wouldn't stoop to what seems like childish antics of "you and my friend don't like each other so I'm not playing with you" and there probably is much more to it than this but there must be something up between those two.
With the exception of the 1995 online chat with CM that someone posted a while back in the "Rumours" board here where she says she thought DM was a great guitar player but "doesn't fit in with FM" (or something to that effect) and the few pictures that were taken I have seen circa 1995 of the "Time" album gang all together (which they are not in the "Time" CD booklet pictures). I've never heard one mention the other in any way.
As for Michael Thompson, (if it's the same one I'm thinking of anyways) he is a session guitarist and I know I've got a few other albums he plays on but I can't think off hand which ones they are now (figures), check
www.allmusic.com and see what it comes up with when you search for him, maybe that'll help.
I'm not sure what Steve Thoma had done before "Time" (and the tour) but he did go on to tour in some of DM's solo bands in the late 90's as evident on the DM & Jim Capaldi live CD "Live - The 40,000 headmen tour" so I'd imagine he came from the DM side of the camp but who knows?
I hope that helps in some small way and I would be interested to see what others think is the deal (if there is any) between these two and if anyone has any other info on Thompson & Thoma.
John