I might be wrong, but I didn't see such approach being used so far.
From EXOS User Guide I see a VSA 'Extreme-Shell-Command', I don't know what is this, it is not describet, from the table on page 939 of EXOS User Guide it seems it is only valid RADIUS response attribute for PAP requests, and somewhere on this forum I found a note that this shall be gone obsolete for a while (it's in the latest docs though).
Theoretically this could be introduced but you should talk with Extreme about feature request, as right now from development roadmap or marketing strategy it might be a minor case compared to enhancing the Policy capabilities perhaps. With XMC you don't have to configure the switch via CLI, BTW.
HTH,
Tomasz