you're not the first person to suggest that to me :) although I haven't thought about using ZFS for this. You're not the only one to suggest ZFS. Why that and not compression in btrfs (or something else entirely)?
_hopefully_ the mmap interface would provide the best of all worlds: mongodb continues to be simple with respect to how it handles getting data from disk and the kernel/fs can do it's magic behind the scenes of mmap. Of course, it could be that mmap + compressed filesystem leads to some unexpected (and bad) perf results. But then again, I've never tried :) have you?