The Hold-to-assign hotkey is something I've implemented in RetroLemmini which can be very useful for levels which require repeat assignments of the same skill.
Essentially, whilst the hotkey is held, any lemming under the cursor will be assigned the currently-selected skill (i.e. if they can be assigned to, and if the skill is available).
Armani has implemented something very similar in his custom NeoLemmix build, which has prompted me to post this topic. I'll look at adding the hotkey to CE soon.