I have just added this feature. Keys F3 and F4 will subtract and add 1 (respectively) to the drawing value.
Source code is available now, and linux_gcc32.tgz binaries. Other binaries will have to wait until I go into the NIH campus on Monday.
To expand a little:
4 keystrokes in the image window now interact with the drawing plugin:
Del = Undo (this is the keypad Del key)
F2 = turn pen mode on or off
F3 = decrement drawing value
F4 = increment drawing value
If you press F1 in the image window, you'll get a popup help window with a terse reminder of the keystroke shortcuts that are available.