anyway.. as you said it won't make much sense to do what you have described... it won't be as efficient as a normal setup and there are no practical reasons to use it
so I think it's a good moment to stop discussing partly closed circuts and stuff
1. monitor the pressure and shut off the pump manually
2. install a pressure switch just like a regular shop compressor uses, when the pressure exceeds a limit the pump is shutoff
3. install a pop-off and just let the system pump and vent or 4.
Figure out a way to recycle from the high pressure to the low pressure side when the pressure limit is reached. Instead of the low pressure side sucking in fresh air the regulator switches over to pass gas from the high pressure side back to the low pressure side
points 1,2 and 3 are ok... but point 4 is more less the same as the partly closed circut idea... it will cost more than points 1,2 and 3 but it won't be any better than them....
AFAIK there are pop off vavles that have threaded NPT exhaust ports so it won't be that difficult to do... but as you and I agreed earlier this idea is not practical...
all in all it will be much cheaper to get an old co2 FE or scuba tank and use it as an air resorvoir and either manually turn the thing off when needed or add a pop off valve... instead of spending extra $ on low pressure tanks and stuff you can buy more high pressure vessels