Welcome, Mal.
Try posting this question in the sister site, http://www.hybridphoto.com/ , there are a lot of folks there that use flatbeds and maybe someone will know the answer?

Failing that, some of the specialty holders for the Epsons might work on the Microtek? I know some microtek scanners use a "below the glass" technique for scannning photos, is that the case with yours?
If so, you may have to home-modify the medium format holder to make it take 645...