i seem to remember reading that true temperment is a temperment (not equal temperement) similar to a popularly used temperment but can't remember the names right now. but a lot of piano tuners are using a version of equal temperment that could be adapted to the guitar. i would think the issue with tempering the guitar is that with different chord forms it could never work out with straight frets. i mean you can temper open strings but the fretted notes don't work out exactly the same in al positions even with special nuts and weird intonation procedures. the only possible temperment to be perfect would be even, but even temperment sounds bad. obviously true temperment necks are an exception but temperment in and of itself can only be a better compromise. there is no such thing are perfect harmony everywhere without synthesis which can also sound bad, as humans we seem to accept some of the imperfections and even prefer them so why go to such engineering measures to make a costly fretting procedure? can we get a better compromise without bent frets?.
there is a technique some piano tuners use that is technically even tempered, or well it's close, it might never end up identical on two different instruments due to the nature of the process, it ends up with a little stretch that gives more near perfect harmonies. instead of using a circle of fifths or a circle of thirds where the octave ends up being off, they use a circle of fifths with an extra octave over the fifth. with this it prevents the mathematical error of the fifth from compounding over 12 notes to add up to a noticable amount by the time you hit the octave, you still end up with stretch, but a bit less. and by making this particular harmonic the point with perfect harmony isn't enough to mess up the fundemental in a way that sounds bad and the technique adapts itself to the imperfections of the harmonic stretch within the strings themselves, if you use thicker strings you end up with a slightly different variation.
basically when a string vibrates it carries several harmonics but when measured these harmonics are slightly higher in pitch than the perfect ratio they should be. thicker strings tend to have more error. a 12th fret harmonic is a little more than double the pitch of the open string, this is 1 part of why there is dissonance in even temperment, the other part is that there are no perfect harmonies in even temperment even if it's computer generated without physics mucking things up, the notes only end up close to whole number ratios not on them. the process of using an octave + 5th seems to do a good job of making a better compromise by making some near perfect harmonies and doing it using the real harmonics generated by the strings. and all that can be applied to guitar with slight nut compensation and by modifying your intonation procedure. no electric tuners in the intonation process. perhaps a strobe tuner if you can apply it properly, but it can be done pretty well with a naked ear listening for beats in the harmony. I start out intonating opens strings to the 19th fret instead of the 12th just listening back and forth. i tune the "D" with a tuner holding it at the 14th fret and tune open A to that, then i tune the open high "e" in to the open "A" then i tune adjacent strings holding the 14th fret on the higher adjacent string. im not listening for the fundemantal, i listen for the least beats in the harmonics, you should get a pretty harmonious 5th like this. if things aren't in check when i get up to the high e I retune the A to a tuner then recheck the open A against the e as tuning may flex the neck and change things. then i can play with different combos that hive the 5th +octave to try to decide where to play with the intonation. i haven't played with nut compensation along with this but it's theoretically possible to intonate both the nut and the saddles this way if you spend the time to work out a technique.