Thinking about this again, having a brass bodied gauge under continuous pressure is probably not a great idea.  Instead, I've decided to install an elbow between the valve body and the gauge which will allow me to rotate the gauge to where I can see it plus I can do this without emptying the bottle.

Problem solved

My 10L air tank has the pressure gauge mounted on the side of the valve block and it also has a rubber shroud.  I don't know how many times now that I've confused it for the valve and tried to unscrew it so I'd like to relocate it.  Currently it only reads when filling the guns as it's tapped into the discharge side of the valve.  It's also hard to view as I fill with the bottle standing.

My thinking is that I could (after the bottle is fully discharged) remove the bust disc fitting and then install it and the gauge into a new tee fitting installed where the burst disc currently resides.  This means I could rotate the gauge to where I can see it and it will always be showing the cylinder pressure.  The burst disc would remain in its correct location, albeit no longer screwed directly into the valve block.

Would this be legit, or are there rules/regulations that would prevent me from doing this?