Harmonics will always be correct if the fundamental is correct, their only use in tuning is to compare to the 12th (or 17th) fret. Changing the truss rod will never make (any appreciable) difference to intonation, although if you are going to do it, do it first.
It is all about the bridge.
Play a harmonic at the 19th fret, then play the note at the 19th fret.
if the note is sharper than the harmonic, ie, the note is higher, it means that the string is shorter than it needs to be, so move the bridge end back away from the neck. if it is lower, then the string is longer than it needs to be, so move the bridge end forwards (towards the neck).
I don't know how the hofner does it but if it is a floating bridge then you just have to come up with a best compromise.