I use the system that you described. The adapter thing that the oil filter screws on to is from an old VW. A standard Fram CH48108 screws right on to it.
NOTE: The only time oil goes through the filter is when the oil is hot enough to open the varitherm. Your not filtering 100% of the oil flow, but in my opinion, this is superior to what you have now. The filter will catch the big chunks, the filter will catch the small chunks. Do 50 hour oil chanbges, but check the screen also.
WARNING: Use ONLY aircraft oil filters. The oil pressure at this point could reach several hundred psi. Cheap automotive oil filters could burst under these pressures.
In automotive engines, the oil pressure regulator is normally an integral part of the oil pump assembly.
Waiter
In lycoming engines, the oil pressure regulator is independent of the oil pump, and is actually located after the cooler. Pump pressure could easily reach several hundred psi, especially when the oil is cold and thick.