I figured it out.
Inlet fitting for the P21 / P0 is 1/4 BSP (british parallel thread w/ o-ring).
I removed the fitting from the filter housing, and put it back on the hardline without the filter in place.
Then, I attached a 1/4 BSP-F to 1/4 MNPT fitting.
From there, I have:
Double NPT female check valve
1/4 NPT to #4 JIC Parker hose whip
#4 JIC to 1/4 MNPT elbow
1/4 MNPT street tee w/ gauge (pre-coalescer)
Parker JN1A coalescer w/ 4CN cartridge (99.995% efficiency)
1/4 MNPT street tee w/ gauge (post-coalescer)
1/4 NPT to #4 JIC Parker hose whip
#4 JIC to 1/4 BSP-M
Bauer P21 / P0 filter
This setup addresses some of the shortcomings of the water P21 / P0 filter. The filter can now be kept with positive pressure in it, so that will help extend filter life. Coalescing will take place outside of the P21 filter housing, so filter life should be substantially extended.
I'm using an OEM Bauer filter (MS13X/AC/HOP version). I'll weigh it once a while so I can figure out filter life. If this extends it to a 1 year change interval, I'll be a happy guy!