Well, just a couple of comments, and please don't take them personally.
You can't justify the mobipocket format's intractability by saying "it supports old devices" or "it's a de facto standard." That's the same logic that Microsoft uses to keep us locked into their proprietary binary document formats (and now proprietary XML document formats). If there are industry standards that everyone else has agreed to it makes absolutely no sense not to follow them except if they're trying to monopolize the market by using standards lockout.
It's incidentally the same logic that gave us HTML hell back in the nineties. Neither Netscape nor Microsoft wanted to play by a common set of rules; instead they just did whatever they pleased with HTML as a standard, and by the end it really wasn't one.
The good news is that the Kindle uses the Netfront browser (v3.3) to render HTML. That should make it XHTML and CSS compliant, so if I fiddle things properly I may be able to find a way to go Docbook via XSLT -> XHTML/CSS. Wish me luck.