Somewhat interestingly, WebDAV has this idea of combining multiple requests (in an XML wrapper). The intention there is to do the entire set of requests in a transactional fashion. If you just have a way of referring to a transactional context, you could then explode that multirequest into a set of normal requests. This doesn't work across multiple backends (without, I suppose, some fancy transactional event system), but at least it offers transactions on some compound structures. (I've actually been thinking about this a lot as it relates to OHM, as it's kind of an outstanding issue there.)
The other approach is to be sloppy, and to be ready to deal with situations where the consistency of the system has been compromised. This is much more practical when you don't control all the pieces. Actually, it's about the only thing that is practical when you don't control all the pieces.