It's not a bad idea, but if there was a drop in air pressure then how would you find the leak? Would it be guaranteed to be making a hissing sound?
I'm just about to overhaul my system so I might try to source some fittings and do this. I actually think I do have a leak in my system and it's coming from 2 rotary fittings which I think are probably the biggest risk in terms of leaks. However, it might just be some condensation due to the temperature being close to zero right now. That might actually be causing the leak as the fittings will be shrinking slightly in the cold. Pumping some air in there might prove that one way or the other.
Anyone got any ideas where to source the parts?