Here's something interesting...
An Atari ST mouse to usb mouse connector. In theory, this should also
work with the SAM mouse interface, should it not? So we could use a
modern usb mouse with the old MGT mouse interface?
http://cgi.ebay.co.uk/Atari-ST-USB-Mouse-Adapter-Brand-New-/120732726502?pt=UK_VideoGames_VideoGameAccessories_VideoGameAccessories_JN&hash=item1c1c3b34e6#ht_2467wt_1139
I suppose the other questions, which others will know better, are
whether or not it would support a)optical mice and b)perhaps even
wireless? Though I would presume there are other issues regarding
power from the USB port, etc, for wireless and optical that might make
it incompatible with the SAM. The item listing seems to imply that it
would work with optical and wireless mice on the ST, so who knows?
An interesting possibility! :-)