Tried this on a 2.0L WRX last weekend. Works wonderfully. I only had to use 12psi, and a bottle of soapy water to find any leaks. Sprayed the soapy water on all joints that are supposed to hold PSI, any leaks were clearly evident by the soapy water starting to FOAM like crazy. 20PSI probably isn't necessary. If you drill a big enough hole for the tire valve to push all the way thru to the ridge, you won't need to use silicone since it will seal itself much like it would on a car rim. Another tip: PVC cement isn't needed to hold the cleanout adapter to the elbow... at 12psi, it wasn't anywhere near popping off and sealed well.
OP & Jorge: Thanks for the info, it was very helpful