They haven't done a book set reissue of MU (or any sort of compilation album for that matter, like Living In The Past) yet, so there hasn't been any reason to release this quad mix - if it does actually exist. That said, the fact that all of Too Old and part of Songs From The Wood - both of which came after MU - were done in quad does make its existence seem somewhat plausible.
A couple years back, @steelydave found an old billboard article dated December 1975 mentioning that "Jethro Tull's Ian Anderson has been working on a quad mix of the band's last LP with Rick Heenan at the console". I guess this could be referring to Minstrel (even though the quad master tape box credits Robin Black), but maybe it was the MU comp?
That's very possible.
It could even be a partially completed quad mix of M U Best Of's .?
For my money I'd love to see the Jethro Tull's Living In The Past get SW treatment. .I think most if not all of that album is currently mixed in surround .
(And both M U 'S vol one and two✌)
We have Ian Anderson and Steven Wilson to thank for such surround material
from the Tull Catalogue.....and they're still not finished!!