thats correct - as long as a player is correctly IDed and tagged - and RPs based on the basic guidelines of the faction, you cannot tell him what ship or equipment to use.
i think not even the RM who were first to attempt a hardcoded rule about it or anyone else can do that. - if you RP it - you are allowed to use it - simple as that. come to that - if 100% of a faction RPs it - they are all allowed to use a battleship ... . but thats also a happy thought - cause it involved that 100% of the faction RPs correctly... so no harm done.
edit: about the rheinland military .... they got the deserved critizism for their strict politics - and i think it was mentioned that there is no universal rule that is applied to each factions freedom to restrict or not restrict things. - some things / quotes from admins are simply contradictory. - some advocate freedom based on the basic rules, others support restrictions in special cases. - and the "real" rules say - you CAN fly it if your tag and ID matches as closely as possible. - other than that.... only the "don t be ooRP" rule applies.