I say let them have one - but without armour upgrades, and some strange mashup of weapons.
To be sure, the nearest comparison for 'stolen' Rheinland gear is the GMG, who in RP managed to get hold of three battleships (only one playable).
But then:
1) They were in RP victors in a war, and to the victors go the spoils.
2) The GMG are wealthy enough to maintain a single battleship.
3) The ship was not 'recently acquired', but rebuilt over the centuries since the war.
I would suggest that these are three conditions that are needed for any faction to acquire the capital-class technology of another faction (without their consent).