E-mail hash is a pretty terrible way to determine account association, as there's no uniqueness guarantee. I touched on why account associations - which is a more general case of what you're asking for - aren't exposed in this answer, but I'll copy it here too.
Currently, we only have two places it could go:
- not "global," and accordingly shouldn't know about all the other sites in the network
- more "global," than the above, but also a site onto itself; it'd be a weird one-off to also have it serve up
a network map account associations
Once there is an appropriate place, the data will be served up from there.
For your specific use case, might I suggest having the user enter their display name & email address? You can filter based on name, and pick the right one out based on email. Heck, it probably be just as good to just go on name, present the gravatars, and then have the user select the correct one.