Thanks for the help, Speeddog, unfortunately I wasn't able to make that page load from Australia (though it's working today, strangely). But in the meantime I made some progress.
According to
this page, the brembo rear caliper on the 1993 M900 is PartNo: 20.5161.42
Which led me to
this PDF which has the breakdown for the caliper itself, including dimensions, and most importantly, the 'available spare parts'.
The bleeder screw is PartNo: 105.3246.11
(the dust cover is PartNo: 205.1502.10)
The bleeder screw is described as 'F8 Style'.
So now I'm off to find a local source for this part. And then I'm going to screw it in
tightly. =/