I'm not certain if anyone else has had as much trouble as I have getting the portable version of Multi Commander to work in launching other portable programs, but I was finally able to compose some syntax that does this well, so I thought I'd share it.
I realize you should be able to do this with the External Command Type, but I was never successful with it. Everything I tried either couldn't find the program, or when the path was successful, gave me "Access Denied". You can always use a batch script, but then you wind up with an open console window until the external portable program closes. The best working solution was to use Custom Commands.
For example, if you keep the Multi Commander Portable directory in the same directory as your other portable software (such as \PortableApps), then create a User Command with the Command Type Custom Commands, and the following command syntax:
MC.Run CMD="${mcinstallpath}\..\FirefoxPortable\FirefoxPortable.exe"
This will work no matter what your folder focus is. If you need to run the program as Administrator, then type this instead:
MC.Run CMD="${mcinstallpath}\..\FirefoxPortable\FirefoxPortable.exe" ADMIN
I've found that arguments are relative to the command path, so you can type a portable command with an argument like this:
MC.Run CMD="${mcinstallpath}\..\AutoHotkey\AutoHotkey.exe" ARG="\Scripts\backup.ahk"
I can now create menus and use Multi Commander as a portable program launcher, increasing its usefulness immensely.