Hi:
I do NOT know of a method to dynamically update a parameter list -- either for a "regular" SP or for one linked to an Info Map.
I know that dynamic parameters are planned for 9.2. Until then, the most definitive answer will come from Tech Support. If you can remember who told you this functionality was available, you might want to check with them and see what version of the Platform they were talking about. Or check whether they had written their own servlet to build the parameter list dynamically. (Which would happen in the servlet code, not in the Info Map or in the SP).
cynthia