Placing an interior other clients don't have will crash them.
Placing new interiors may cause them to be unlit (and appear totally black), and for the scene to be relighted every time.
Placing lots of interiors, which may happen if Mirrors are available as bricks, would cause the scene relights to take longer and longer.
Wait for engine changes or something to have these available as bricks. I can't guarantee it will happen at all (I know nothing about Blockland's future any more than you do), but it's the only way it could happen in an efficient or non-laggy way.