My thinking exactly, although my version was to core the ET like an apple, plug a Bigelow balloon through the hole with an airlock mount, and inflate the balloon inside the tank. The balloon could be pre-fitted with attachment points for internal dividers. The space between the balloon and the ET wall (occupied by the anti-slosh baffles) could be filled with water for additional radiation shielding and general protection. Solar panels could be mounted on the ET structure. It'd be a beautiful thing.

They'd also make great storage compartments or extra space, hooked up to a universal docking node, a hab module, and a couple of CEVs for those long duration trips out to the NEOs.