Saliva may well play a huge part in destroying the HIV virus, but so does air, water and just about everything else - it's a pathetic weedy thing compared with the likes of the herpes virus, or the gonorrhoea bugs. I stopped doing OWO around two and half years ago, but I can honestly say the risk of HIV never even crossed my mind when deciding. From what I do know, if you were unlucky enough to catch it from unprotected oral (assuming you have good oral hygiene and dental health, and you shouldn't be doing OWO if you don't) you'd more than likely be run over by a bus on your way home from the clinic.
What you can catch, saliva or otherwise is pharyngeal gonorrhoea (the most likely), chlamydia, far less likely, and even less likely that you would pass it on if you did get it or syphilis (so contagious that it's apparently almost 100% that you would catch it in the event of unprotected contact with someone who had it, but it's a lot rarer than it used to be - I think it did enjoy something of a renaissance amongst gay men a few years ago so anyone seeing MSM might do well to bear it in mind).
You (or punters) can catch the mostly the same things from unprotected reverse oral but the likelihood is far lower as infections like chlamydia tend to be concentrated around the cervix, which isn't really accessible during oral sex. To suggest that the risk is equal because the range of infections you might pick up is the same is like saying the risk from OWO is the equal to that from unprotected penetration for the same reason. Source: my local clinic, who I have grilled half to death over the years, and the Chester Health website.