Neck relief issues...

[1337storm]

Has it been refretted at some point in time?
This is a bit of a longshot, but if you do a refret and have either to narrow slots on the fretboard or to wide tangs on the on the frets, the result will be
a compression in the board that is impossible to fix without doing it all over again.
It doesnt need to be much compression on each fret to cause a permanent backbow, cause it's times 21, 22, 24 or whatever and it adds up really quick.
So if all else fails, I would remove all the frets and see what happens, it's only a few hours work to do a refret anyway.

[straycat]

I had one neck thru guitar that had no relief and a loose truss rod nut.
I left the truss rod nut loose and put a 13 gauge string on the high an low E and used the other strings on it.
Tightened the strings to pull the neck up and put a little bow in it.Left it for a few days checked it not much movement.So tightened the strings a bit tighter left it for 3 days checked it and I had some up bow.
Tightened the strings some more and let it sit one more day and it had what seemed to be enough.
Loosened the strings and the neck had a nice up bow.Put some 10' on it and tightened the truss rod nut and its been fine ever since.

[j2379]

Any luck yet? im telling you what i suggested works, ive done it with good results.

[DalyTek]

I ended up trading it back in. Everything I tried did not help. Heavier gauge strings did pull it forward enough to eliminate the buzz (with the truss rod nut completely slacked off) but the slight forward bow wouldn't hold.
Thanks for the suggestions everyone, but I honestly didn't like the guitar's look enough to warrant any more effort.

[Axewielder]

Honestly that was probably a good move.

[straycat]

I think you did the right thing.

[Devotee]

I think you did the right thing too. Another possible solution might have been to shim the nut and raise the bridge slightly (assuming it had a Floyd) so that the strings cleared the frets. Unless there was so much backbow in the neck that doing so would have just made the action a mile high.