Well, assuming I get your description correctly, you could drill the tank, use the external pump (placed on the same platform as the sump), and just use a pvc union above the pump. That way, you simply disconnect the union, and pull out the platform as usual. Then when you push it back in, make sure the pvc lines up, reattach the union, and your good to go.
I think you'd have to do that either way you go, whether you use a submersible or external pump.
I think you'd have to do that either way you go, whether you use a submersible or external pump.