Follow up from issues raised here: https://groups.google.com/d/msg/datomic/blgLZkYF5xA/LFjGCASHZNUJ
N.B. These experiments were carried out on a restored backup of the DB (so therefore backing up and restoring isn't a viable solution).
Keywords appear to get added to DB values like so:
user=> (.addKeyword db :this-is-a-test 999999999)
datomic.db.Db@d37f7c59
user=> (d/ident *1 999999999)