Yep, last night I ran the scenario with the Hack 'n' Slash chart for HARP without BRP special/crits.
The nice part is that since HARP is also a percentile based system the modifications for target size and some other bits pretty much port straight over. And since HARP is a fantasy game with "funky" creatures, the crit descriptions aren't perfectly nailed down to bidpedal, two armed targets.
I'm going to do a bit more exploration with the various crit charts from Iron Crown; standard HARP, Hack 'n' Slash, Martial Law, Rolemaster Express to see if any one of them stands out through play.