I wouldn't be surprised if he had multiple sends going to the PA, pre and post processing. At that point he had worked out a small PA on stage, so a direct line would pretty much sound the same through his amp or through the mains.
My guess is that he has been going direct since at least the mid 70s. When the Alembic input modules, the IN-2, were developed (ca. 1973?), they included DIs, so I'm guessing that they were used for Phil's bass even back then. I know he also went direct on some of the earlier albums, but that may not have much to do with live sound. OTOH, the Wall was definitely direct!
In fact, in the very beginning when Bear started messing with everything in the mid 60s, all the band's instruments went to a central processing system and then were redistributed back out to the amps and speakers (including a big rubber subwoofer), so everyone was direct. With much delay of game and other hilarity ensuing. It was one of the reasons they parted ways with Bear for a while.