I would say it depends on the [future] AfriNIC procedure. I mean if the objects will be transferred not all at once but gradually there may be a situation when some objects still have to be kept in RIPE DB, and other (transferred ones) should live in AFRINIC DB. Obviously transferred objects are subject to future change, so they will have to be synchronized or removed from RIPE DB. In this scenario the object immunity can cause needless trouble.
Isn't it reasonable just to change the maintainer to the another one, with the unknown password?