Is it legal/ advisable to directly wire a pneumatic cylinder after a regulator so that it is continually supplied with air pressure, without a solenoid? It would be used to apply a constant pressure to something.
I see no reason why it would be illegal.
But why not just create a rigid mount; isn’t that essentially what you’re doing?
I’m sure you’ve thought this through though, and you know whats going on and have a good reason.
I think you give me too much credit. 
It was an idea that was bounced around15 minutes before shutting down the build for the day.
Yes it is legal. We use it to make an easy shock absorber that is fully adjustable.
You can also put a solenoid to the other side and when equal pressure is applied it allows gravity to move what you are controlling.