I've received a response direct from Asus support, and they shared that 128GB is, indeed, supported on the RVI Apex, along with recommending that I'm on the latest BIOS (naturally).
I'll be purchasing a 128GB (4x32GB @ 3600MHz) kit from G.Skill to see for myself, but if anyone stumbles upon this thread in the future, and there are no further updates (I may forget to come back here and update the thread after a full system tear down in a few weeks), then it is probably safe to assume that the board accepted the DIMMs and all was well.