Related to this, In 32bit Windows7, /run -a fails to launch 16bit DOS executibles.

/run c:\windows\system32\ /k
/run -a c:\windows\system32\ /k

If mIRC itself is running as admin, the target program runs in admin mode without the -a switch, as happens to all programs launched from a client itself running in admin mode.

workaround success:
/run -a cmd /c c:\windows\system32\ /k