Allowing any permanent asset to be attacked while the owner is offline is stupid. Either they're so indestructible they might as well not be damageable at all, or they're guaranteed to be destroyed so they might as well not exist. There is no middle ground.
I don't see any reason why these couldn't exist. Lets not make up technical reasons when we have no idea how the game is coded, eh?
I'm tentatively in support of an idea like this. It should cost around 1.5b, and it should only be able to carry 1 large, 1 medium, and maybe 2 small ships. Limited cargo capacity - 500-1000 at most. If people want one of these, able to get in and out where FCs cant fit, more power to them.
I'd especially love it if I could own both, and have them both in the same instance, flying in convoy.