The answer to the quota question points out that this call is only the first step.
There is, of course, also a need for reflinkstat() or whatever it is going to be called - one needs to find out how many blocks are part of a specific COW, and also reflinkdiff(), which enumerates the (meta-)data blocks which differ from the link target (needed for efficient rsync/backup).
Then, it also becomes possible to use quota to account just the difference, i.e. the actual space used by the reflink.
A further complication arises when we look at using reflink() on directories, which would of course also be quite desirable (for snapshots, multi-use chroots etc). That will be an interesting direction to explore ;-)