I'm spending my day looking at varios front ends, and MI is currently a front runner

due to that I haven't actually installed it yet, but this should work...
open notepad and type
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
save this file as minimize.scf (or anything you like with a scf extension) add an external application in MI that points to the scf file ... this file is the same as the windows show desktop button from the quicklaunch taskbar.
You can also just find your quicklaunch folder and run the "show desktop.scf" file from there ... wasn't sure if MI could handle the space in the file name though since I haven't tested it.
-Aaron