If you hg push changeset A into repository myapp-v1 and then do an hg pull and hg merge to merge that into myapp-v2, is there a way in the myapp-v2 repository to identify that the changeset was originally checked into myapp-v1?
No. DVCS is DVCS and commited changeset haven't any origin-data after transfer to other clones
Workaround - permanent attributes of each changeset are
you can use these signs