On with the fix! It's simple! Just enter "smit keyboard", select "Change/Show" and change "extended keyboard identifier" from "none" to "nonum" (you can set the keyboard repeat rate here too!). Then reboot.
The numlock function then works as described in the user manual: press shift-numlock to turn it on and shift-numlock again to turn it off.
This should work on at least AIX 4.1 and 4.2. Email me if you have a problem.
A bit of history: On a hunch and advice from Andrew Yeomans I started messing with xmodmap to see if I could manually insert the seemingly missing functionality. After messing with that at length it seemed futile to try to make a new modifier for numlock because it seems X wants the mod2-mod5's to be valid-while-depressed modifiers like Alt and Control and not "locked" ones like ShiftLock. Perhaps Andrew was correct in his assumption that the numlock functionality would have to come from lower down in the system at the device level.
On a whim I started plunking around IBM's web site and behold there was an APAR about this exact problem with the nice solution I rehashed above! Congrats to IBM! For those interested the APAR is IX62290 and should be available at: