A thought for you. Let me say straight away that this really only applies to MFR, Rapid 2 and late Rapid 1 owners for reasons that will be obvious as you read on.
What is the most important feature that a standard production Rapid lacks? Maybe there are several answers to this but to me it is simply the inability to see at a glance that your air supply is OK before you take your rifle out or whether it has sprung a leak in your cabinet? I miss not having a pressure gauge and yet it is quite easy to fit one. If you look underneath the block of your rifle you will see a circular blanking plate that can be removed and replaced with a small pressure gauge available from suppliers -Best Fittings would be my chosen one but 'other suppliers are available'. It can also be used to fit a QF nozzle but that isn't any good if the rifle has a regulator. If you want a QF nozzle and regulator there are several combined units available.. The problem with this option is that it is expensive.
The only drawback to fitting the gauge-or combined unit for that matter- is that you will need to cut a hole in the stock to accommodate the gauge as it protrudes from the base of the block. Provided you use a decent quality hole cutter this isn't a problem. My tip when fitting the gauge is to drill a small pilot hole from the INSIDE of the stock in the middle of the recess in the base of the stock where the original blanking plate sat and then to cut the main hole starting from the OUTSIDE of the stock which should be securely fastened in a Workmate or soft jawed vice to avoid damaging the stock. You will need a 25mm cutter to do this. Other than that no other modification is required. My own legal limit Rapid has the easy fitting 'MFR' type regulator and the gauge as described and it works well. Using this regulator system I really don't need QF and means I can take several buddy bottles out shooting if required. I can see at a glance when the bottle needs to be changed. Just be aware that on a regulated rifle a gauge in this position measures the pressure in the block itself-in other words the working pressure NOT the pressure remaining in the bottle itself. Assuming you have your regulator set to 100BAR working pressure you know you have to refill if the pressure falls below this level. Personally I check the working pressure every time I change magazines which pretty well avoids the need to count shots.