Hmm. Further to the thought about sub-letting reserved space until it's required.
While some things that go into that space may be permanent additions or changes, it seems likely that a lot would also only be mid-long term or of niche appeal. In that case they'd be functionally identical to sub-lets for user groups or members.
So maybe it would be better not to make sub-lets a temporary addition, but a permanent feature to accommodate mid-long specialist workshops.
EG; a darkroom is a commonly requested feature of a makerspace, but it only used very occasionally by a small proportion of the membership. Allowing a photography group to rent out a room off the main workshop would add the feature at no risk of wasting resources on something little used.
Another might be high-vacuum equipment. Surface science & space research. It's a specialist interest and may only been needed for an 18month project.
Sunbeam City is a anticapitalist, antifascist solarpunk instance that is run collectively.