Client update: introduced ctrl-G for toggling the backpack and ctrl-R for toggling the toolbelt window.
Unlikely to work with all toolbelts (as it looks for named windows containing 'belt' in their name) but should work with all backpacks.
If you have any belts that do not work with the feature (every one of them will open, but not all may close), let me know and I will patch it in.