Thanks for all the input. The original neck measures out 63.75MM and has (as noted above) a pocket-end truss rod turnwheel.
I think a replacement neck with a neck-access truss rod adjustment would keep me from having to muck around with it.
I was thinking about something like this:
Yellow 20 Frets Maple Bass Guitar Neck for Precision P Bass + Truss Rod | eBay
Remember, it is a 'value' instrument and I'm not going to put a $400 neck on it.
Though this listing says that neck is 64mm I suspect that it is not exact. I also think that I could come if with .25MM if I had to.
More to the story: I picked this bass up for a good price because the neck was bowed. The TR seemed to work and I wondered if maybe the seller just didn't know how to adjust it. Upon getting it home I tightened the TR as far as it would go and still the neck was quite bowed. I did find that if I loosened the TR all the way I could slide it out of the neck completely. Upon inspection it seemed that it worked properly.
Figuring I had nothing to lose, I headed out to the shop. Using my ski iron and a putty knife, 90 minutes of labor got the fingerboard off the neck.
I saw that the neck end of the TR slot was enlarged a bit but it looked like it might've been made that way (the jagged edges are mostly dried glue). I saw that the wood under the TR end was compressed a little so I glued in a piece of plastic to bring the bottom of that pocket level with the rest of the TR channel.
Using various shims and clamps I made a jig that would hold the neck flat when clamped. I installed the TR and then re-glued on the fingerboard and clamped everything down.
After the glue dried I took it for a test run and everything seemed good. Neck held a proper relief, TR is working and still has some turns left in it. I dismounted the neck and cleaned up the dried glue, sanded the sides a little to re-meld the FB to the neck, leveled/crowned/polished the frets, and gave the back of the neck a light spray of matte poly (then steel wooled that smooth) and the neck looks new and is likely more leveled/setup than it would have been new (being a budget instrument):
I originally got this bass with the thought of upgrading the electronics but I never got that far because of the neck issue. Now that the neck plays well I find that the thickness near the nut and the 38mm nut width is too small for my liking - My P-bass (my main instrument) has a very chunky neck that's 42mm at the nut and even my other J-style, a Yamaha BB604 is 40mm and thicker front to back.
So now I'm a bit torn about what to do with it. I could get a similar neck to the listing I included above but that is 42mm at the nut I guess. Or I could just sell this bass as it is now - it'd be a great starter bass or a good backup. Has anyone had experience with these $50-ish 'Candian Maple' necks off of EBay?