just read on Utube that the guy named "delawarecop" there on YouTube has used a "CAT" RPCV to replace the Isuzu part successfully on a 4JX1. He wrote "cost me $150 landed and I have another 10 units coming". If he sends me part numbers i'll update here - i've pm'd him there. I guess he hopes to sell a few, but given the research and experimentation it takes to test these things, fair enough. Either way, interesting to know that Isuzu took the CAT patent on oil pressure regulated HEUI injectors seriously enough to use the same part.
I have no idea if this works, but it seems logical. I hope he replies to me because for $150 it isn't worth taking apart the valve on my Jack.
This is in no way an endorsement or recommendation. I just read it. But i'll be following it up myself.