More on the Aquarius…
I found a site which has a disassembly of the Aquarius ROM, as well as some other technical info and an emulator. Looks like I have a bit of reading to do! 
As I mentioned in the previous post, Mattel disabled PEEKing into the ROM. The author of this disassembly wrote a small machine language routine which copied chunks of the ROM into RAM so he could dump the ROM contents. This was what I was planning on doing, but now I won’t have to.
At least now I have a list of BASIC keywords. Appears to be a reasonably standard set of commands for an 80’s home computer…