I actually managed to get imwheel to support a double-click mapping to one of my extra buttons. To do this, though, you need to be sure your X server supports the keypad mouse. To find out if it does, press Control-Shift-NumLock. Then press one of the keypad numbers to see if the mouse moves on the screen. If it does, you can create a mapping in imwheelrc to support the double-click button on the keypad (+). Turn off the keypad mouse first, though, by pressing Control-Shift-NumLock again.
In your imwheelrc file, you will want to add the following line (suitably modified for your preferred button, of course):
None, Right, Control_L|Shift_L|Pointer_EnableKeys|-Pointer_EnableKeys|-Shift_L|-Control_L|KP_Add|-KP_Add|Control_L|Shift_L|Pointer_EnableKeys
Then, restart imwheel and test to see if it works for you. To get imwheel to work properly, I have to start it with
imwheel -k -b "4567"
Hope this helps,