>> This information needs to be exported to userland in two different
ways:
>> - A simple list of files and directories that have been updated since 
>> the generation requested. This should be suitable for feeding into 
>> rsync.
The way I read that, it's not something rsync-specific.  It sounds like 
btrfs would allow the generation of a list of files, which rsync (or tar, 
or zip, or whatever) could accept.  (Rsync obviously being the best 
choice, :-) )
e.g.:
$ export-whatever-from-btrfs | rsync --files-from=- / backup/
(or)
$ export-whatever-from-btrfs | tar -Jcvf backup.tar.xz -T -
$ export-whatever-from-btrfs | zip -@ backup.zip
...etc...
> Is this someone that is likely to happen in the for see able future?
Sounds like a question for the btrfs folks.
Best,
Ben