I've sorted this one out today. Essentially re-purposed the code for the NL skill panel frameskip half-buttons and applied it directly to the left and right mouse buttons instead.
It took a bit of tweaking to get the values and time checks right, but it works a treat: clicking anywhere but the skill panel/minimap and holding the mouse button results in near-identical behaviour: a single frameskip, followed by a wait of a couple of hundred milliseconds, then repeated frameskips than run at a good enough pace to facilitate proper use. The game must be paused for it to take effect; left mouse button skips forward, and right mouse button skips backwards.
An added bonus is that you can assign a skill to a lemming and then just leave the left mouse button pressed to trigger the repeated forwards skips (again, as long as the game is paused).
Whilst it's great to have this sorted, I do have some reservations. The fact it's now hardcoded to the mouse buttons means it's no longer sensible to assign a hotkey to the right mouse button, so this option would have to be removed from the hotkey config. Also, since it's already possible to simulate the panel button behaviour by pressing and holding a -1 / +1 timeskip-configured keyboard key, I wonder if this feature is really necessary anyway.
Making it optional is, of course, always an option, I suppose... but, I'm currently trying to de-clutter things, so this wouldn't exactly help with that given the scope of the feature and the fact it also needs to mess with hotkey options.
So, I could do with some feedback on this: do you want to be able to configure the right mouse button to something other than a frameskip? Does pressing and holding the mouse buttons as a way of performing repeated skips (rather than repeatedly clicking the buttons) sound like better behaviour to you, or would you prefer to click each time you want to skip?
Any and all thoughts on this welcome. I'm about 50/50 on whether to let this feature through, even if it was nice to get it working.