Thanks a bunch!
Ok so it does work, but it's not working the way I was expecting.
Your sample works and I get the returned data. But when I try this, it returns immediately with a blank string
if RunCommand('AROS:Utilities/Installer', ['APPNAME','test','SCRIPT','AROS:Marcus/Downloads/Installer-43_3/Installer-43_3/SampleScripts/install-test'], ret, [poWaitOnExit])
If I run the command in a shell it works fine and pops up the installer window, but I don't get anything when trying to launch it myself.