> I'll start experimenting now but I assume TP will pass the name of the file that is clicked on to the calling program?
Yes, it will be put on the command line. So, for your batchfile to pass it on, I guess you'll have to use "%%1" on the command line of the program that you're calling.
> And the option to allow multiple files will let you select multiple thumbnails and pass all those filenames to the calling program?
Yes, but be forewarned that there was a bug which may or may not still be present.. ThumbsPlus should put all the filenames on the command line within quote signs, separated by spaces ("%1" "%2" etc), but someone reported last year that the closing quotes were being omitted. So, if it shouldn't work for you, that might well be the cause.
ps: Note that each filetype definition also has an Editor that you can specify for that filetype. You launch that editor (which can really be any kind of program) by pressing Shift-Enter (instead of just Enter, which would open the image in the internal viewer). So, for those users that prefer to use the keyboard instead of the mouse, that might be yet another way to load the image under the cursor into a 3d party program.