Fancy hotel ladies toilets have power points - for the hairdryers of the beautiful people I suspect. So electric pump-and-dump is pretty simple. And you don't have to empty completely to get a couple of hours of comfort. If you're not keeping the milk and you don't have to be hygienic, then electric expressing can be very quick - get a couple of huge zip-lock bags for the pump bits and just rinse them between times (forget drying or soap or fridging or any of those luxuries).

I used a big bushwalking backpack for conferences - it wasn't very stylish, but it could take the pump and pump bits and the laptop and the spare top (not sure about the black tie attire though!) and so on. And it was much easier to carry on the train than lots of separate bags.