Ian,
This is an arbitrary limit that is trivial to increase. Look at the Interpreter method #userPrimitiveResponse and it should be obvious how to increase the number of arguments that can be passed.
I did look there, the reason I asked was I didn't believe it to be this simple so I wanted to be sure I didn't miss something tricky. And now that I know there are no hidden traps to modification, I take my words about UP being less pleasant to deal with than SP back!
The UP support is unofficial, unsupported, and (originally) specific to Unix
WOW!!! Is there anything in Squeak that is supported or official?!!!
Reagrds, Boris