Was thinking of something else that would require messing with grenade priming: Flares, since the AI doesn't react to light this would only be useful as a tier -1 version of the electro-flare(providing an opportunity for a research requirement for the electro-flare). My thought is you set the timer, perhaps with a reducing the max you can set it to, and it functions same as an electro-flare until the timer runs out at which point it goes dark. It would require the flare to be tagged as used/not recoverable once primed too.
Alternately I was thinking this might be possible to add using the same functionality as the ink-smoke bomb from TFTD, prime as normal then once it goes off it produces light for a few turns the same way the ink-bombs produce smoke for a few turns.