Calum,
When the MSM extended tool table attributes were invented they were used only from within MSM itself - so I didn't create any user visible interfaces to control them. Currently the only way to change them is to alter the XML that is used to store them (and I prefer not to describe that mechanism very much as it is a kludge that was adopted to get more attributes in mach3 where the table was fixed)...
If you really need this ability, I could think about creating API interfaces for the extended attributes.. you;re the first to want to access the info from a script.
Dave