I know you can use a Mach 4 API to run a windows script file. I guess you could script the eject USB function into a windows script and then map the Mach 4 LUA script to call the windows script when the button is pressed.
It might be more effort than it is worth. The remove USB button doesn't do anything more than check if the drive is in use. As long as you aren't writing to it, removing the USB without ejecting wont cause problems.