I really need a dreadnought ship hull type so it's possible to have a carrier bristling with weapons. I'm not sure that you can do that effectively with a battleship hull.
Kevin Crawford has stated (Skyward Steel iirc) that it is deliberate that no ship can do everything, and any custom hull that can is probably too powerful...( unless it is signiicantly higher tech than TL4) . With a fighter bay worth 0pow and 2mass, and a frigate bay at 1pow and 4mass I think it can be done on a battleship hull, just don't expect it to go toe to toe with a battleship designed to go up against other capital ships.
Or you could just slap an engine on a large station (OMG!!!)
I suppose a battlestar or a GSV is a bit beyond the scope of the setting.
Oh my setting most certainly has room for things like General System Vehicles... but I lump them in Tech Level 6+ and so they just don't have/need stats.
However statting up something like the assault ship the protagonists have in Consider Phlebas totally works within the rules
Banks is my favourite scifi author btw.
It's true, Culture tech is maɡic tech and should be TL 6. And Iain M Banks is definitely a favourite of mine.