The magic is 27/64ths

So the magic on the adapter between the 1/8" pipe and the 3/4" pipe is the 27/64ths drill and the 3/4 inch pipe thread to 3/8 inch compression reducer.

1. Remove the compression ring from the fitting
2. Using cardboard or a couple small pieces of wood to pad the compression ring, clamp it into either a vice or some vice grip plyers or find some other way to hold it
3. Cut a slot in the ring perpendicular to it's circumference using a hack saw or a coping saw, the slot can actually be wider than this
4. Drill out the fitting with the 27/64ths bit
5. Slip the split ring over the 1/8" pipe
6. Apply some kind of grease to the compression ring in case it needs to be removed
7. Slide the compression nut over one end of the pipe and the drilled out fitting over the other one
8. Tighten it up

Now you can loosen it, adjust the position of the jet, and tighten is back up. 

Once it's up to about 10 foot pounds it centers nicely. When it's just snug, you can wiggle it around a good bit as if it was a ball joint with limited range. An improvement would be to use a counter sink to restore more of the taper removed by drilling. 

Cost wise, this might be a bit more expensive. The fitting is like $15.00. An iron 3/4" plug, a couple of taps, and a couple of screws is probably $5.00 less.