Delved into this a bit, and found some claims that there's no PPP driver for A/UX, so I guess there's the answer. I know about NetBSD. However, in my somewhat irrational mind, it lacks the cool factor of A/UX, the spiritual stepfather of Mac OS X (the biological one being NeXTStep, obviously).
The PPP protocol isn't very complicated, how easy it would be to add to a binary-only UNIX is another question. You could have a look for whether the A/UX network stack is from BSD or System V. One thing that would make a big difference is whether it supports STREAMS [1].
A few minutes of googling make it look like SLIP and PPP solutions exist for A/UX.
Pretty sure from my college days it has a full tcp stack, certainly I remember running X11R3 on A/UX and using two machines to get decent performance (one as server, one as client)
I used to have my RS-6000 (connected to my Intergraph monitor and a proper Model-M keyboard) running as X client to my laptop, which had a much smaller screen, and work that way. I could run a browser on the laptop (Firefox doesn't like X too much) and have a couple terminals and Emacs on glorious AIX flavored CDE.