Something I just noticed yesterday after reading Bryan Hartnell's post-attack interview transcript with Napa Sheriff's in the hospital was his physical description of the UNSUB: 225-250 lbs. Somewhere between 5'8"-6'0. Bryan probably got the closest and longest look at the UNSUB's frame also with the advantage of it being in broad daylight. He mentions in the interview he's bad with height since everyone just seems roughly shorter than himself. That weight range though, given the UNSUB's estimated age range, is something like 90th percentile-- really closer to 95th+ at the higher end (at the time.) I've never seen that explicitly pointed out before but that seems mildly interesting. My only point here is that whoever Z was he was most likely (and somewhat uncommonly) a fairly heavy-set guy.
Another thing I notice is people pointing out the significance of the UNSUB appearing to have hair and glasses. Aren't these pretty much the easiest/most natural pieces of disguise someone could possibly wear? Didn't the UNSUB himself also imply he was disguised? And wouldn't it also independently stand to reason that he was? My only point here is I don't think a suspect's lack of hair or glasses should necessarily disqualify them.
Further, John Douglas' profile (I know, I know-- pseudoscience etc) is of an offender who needs to seek credit for his crimes. It seems questionable to me that the real Z would allow his identity to be left somewhat ambiguously pointed at the wrong guy in the court of public opinion. But who knows. I wouldn't bet the farm on Z's behavior being completely predictable and he could've also been dead by the time Graysmith's book came out in 1987. He didn't exactly lead a low-risk lifestyle.
Lastly, Douglas only goes so far in his book to say that he believes any good suspect in the case would've at least shared many of the same qualities as Graysmith's suspect. I also agree with you that there just isn't sufficient direct evidence to claim this suspect was guilty of any of the Z activity. I question whether or not that means he should be completely disqualified as a suspect but I'm also sure I haven't spent nearly as much time researching the case as others.