As of the next update (2.8.5), when testplaying a level, it's now possible to generate lemmings on-the-fly using the following:
[Left Mouse Button] generates a left-facing lemming
[Right Mouse Button] generates a right-facing lemming
[Ctrl] generates a regular lemming
[Ctrl + Shift] generates a Neutral lemming
[Ctrl + Alt] generates a Zombie lemming
[Ctrl + Shift + Alt] generates a Rival lemming
(https://i.imgur.com/AEZrEMx.gif)
This is for testplaying purposes only (i.e. via F12 from the Editor), so won't work during regular play. However, it should make it a lot easier to test, debug and backroute-fix levels!
(https://i.imgur.com/SaMwSia.gif)
Implemented in commits 2f08727 and 1c67464.