Earlier in this thread I said I would eventually post everything about this project in the "Hydraulics" forum with a parts list, etc.... That never happened, but I did take a few more pics so I'll just add them here in case some think they might want to go down this same road.
Note: This project was neither cheap (probably a little over $500) nor easy (for me at least.) Many people have pointed out to me that the quick, easy, and inexpensive way to plumb a grapple is to just run long hoses from the rear remote to the front of the tractor and use it. I totally agree, especially if your rear remote control handle is anywhere near your FEL control handle.
For me there were 3 main advantages to my solution:
1) You can use the rear remote for something else -- really important if you only have one;
2) Removing the FEL is exactly the same as before with the addition of unplugging the electrical connector form the diverter;
3) It's nice to have full control with the 6 functions on one lever which makes detailed adjustments easy.
My homemade hoses from the diverter to the original curl/dump steel lines were too short (see pics earlier in this thread), so I had to make them more flexible by creating loops.
I attached the diverter to the loader frame here with bolts. They do not interfere with the loader arm.
Here's the NOS shift knob I found on Amazon to divert the flow to the grapple circuit and some of the wiring detail. The cover is actually pretty nice. I used heat shrink tube on the shaft to protect the wiring to the button.
Finally, my homemade stainless steel bulkhead and my 48" WildKat grapple.
It was a lot of work, but I'm really happy with how things worked out. The hardest part of the project was getting all the right fittings together. The illustrated pics in some of the earlier posts above should help other DIYers avoid some of that.