I compared the AROS BCPL runCommand routine with the Kickstart 1.3 equivalent at address $FF48E6.
The D2 argument to runCommand (stack size in longwords) is used as an argument to getvec. So the actual allocation length is D2 + 1 longwords. You need to addq.l #1,d2 on entry to the routine in the AROS version.
runCommand is not void. It returns the value that the command returned in D0, in D1. (The current AROS code does do that; it just says void in the routine comment.)
Fixed in r44782 - please verify