Ok, having now had a gander at the MM website the solution should be easy.
The fixing brackets appear to be rivetted onto the pannier, the MM frame is 18mm, same as TT.
So, just ask MM if you can buy the panniers without the fixing brackets fitted. (Or probably better with just the top 2 fitted as they appear to have to go at a set distance from the top of the pannier due to the locking mech.
Then just fit the other brackets to the panniers to suit the TT frame.
https://www.metalmule.com/index.php?...=985&Itemid=26
In the photos above you can see that you could fit the bottom 4 brackets to suit the TT frames quiet easily.
Job done!!
Nick.