I have been thinking about this some more and if someone had the know-how I think it would not be too difficult to make .. if someone does, can I get royalties?!! LOL
Electronic leg bands! Chickens allowed in have one type of leg band. Chickens not allowed in, another type and if no leg band then no access which keeps the wild birds out.
When the broody was no longer broody, you could swap her back to an access leg band.
To combat the issue of not being able to access when roost time comes around and everyone needs access, the 'access dependent on leg band type' could be set to only take effect between the hours of say 6AM to 6PM or whatever and anyone wearing a leg band outside those hours would be allowed access.
What if the broody needs to get in the coop to be safe from a predator or something? Then you're stuck!