This would indeed be a great feature, and has been suggested many times. I'm actually surprised it hasn't been implemented yet, as it seems like it wouldn't be difficult to add.
Often, I'd like to part, but still review the buffer (without opening the logs). A simple "/part -k" would work. Or, access to whatever event actually closes the window would be great. I've tried using /halt in the on (^)PART event and parting via a raw server message, but this fails.
Please consider implementing this long-desired feature.