Yes, actkbd solved it for me.
A how-to that may help:
1) Disable (as far as X is concerned) key 123 with xmodmap
#(rough description)
This is probably not necessary if you figure out how to "grab" the event with actkbd. I did not. If the key is mapped to any symbol by X (in my case the 'section' symbol, what on earth?!) and it is not grabbed, then it will still produce that symbol as well as the mouse event. This is not desireable.
2) Map 123 keypress/keyrelease to xdotool mousedown/mouseup
Code:
#File /etc/actkbd.conf
<keycode ("123")> :key : :xdotool mousedown 1
<keycode ("123")> :rel : :xdotool mouseup 1
# NOTE!! <keycode ("123")> is not the keycode reported by xev. You may find the code to use with actkbd by running
# sudo actkbd -s
# and press the physical key on you keyboard.
#NOTE!! LOL
#Also I found that the daemon stole my keyboard interface sometimes, making it impossible to communicate with the #machine, including killing the daemon. It helped a lot to run a watchdog script to 'killall actkbd' after a 20 second #timeout, until everyting is set up ok. But those who know how to use this daemon correctly probably don't need to bother with this
3) Start actkbd daemon
Key 123 now behaves exactly as left mouse button (although a lot more accessible, quiet and easy to press!)
Success!
EDIT: This is a very ugly how-to. If I can help with anything, please let me know.