BTW, the first stage reduction on LP is usually between 90-125 PSI. You can cut an old regulator or drysuit hose and attach what you want with hose clamps etc. The older first stages (like 30 - 40 years old) were eassily adjustable for LP pressure, not certain about the newer. But you can make a small manifold incorporating a small pressure regulator to run the bottle to and then hose off of that for whatever you want. Ya don't need 125 psi for negs, that would just waste air.
There also used to be brass fitting available that fit an LP port on the first stage with a male O2 on the opposing side. You can then adapt O2 to 1/4NPT or use an old O2 hose from a weld/cut rig (green one, red is acetylene with a left hand thread) and then adapt O2 to 1/4NPT at the end of that or ..... endless possibilities.....
Commercial divers have all that crap in their junk boxes, go visit one. Approach confidently with a smile and a roll of good duct tape as a peace offering offering, sea dogs can smell fear too -- but they luv duct tape.
And don't forget about 'Hookah' rigs. Many compressors now are oil-less on the comressor head side. In the old rigs we had to use special oils because of CO if the rings leaked. Been there too.