Even if I could use a Mini/Axe reg I probably wouldn't, for personal reasons, but for what I'm trying to use it for it's just not going to cut it anyway. Palmers makes a reg/bucket changer combo but it's difficult to adjust (has to be adjusted with the 12g out meaning every time you want to make an adjustment you have to de-gas) and I'm not sure how well it would perform all the way down to 190psi-ish or less. They're more intended for LP Phantom and Sheridan setups that don't go much lower than 450 psi.
I like how this looks so far, especially because it minimizes the amount of parts I would need to buy. I'm really only about $50-75 in parts away from having something workable here. Well, $50-75 and quite a bit of machine/tinkering time.