Do note that due to how the game and modding work, all clients will need to know this info. Meaning it being sent over the network. Meaning that reading key state isn't going to happen. I think that your best bet is an extension of custom inputs to allow them triggering on key release. That said, I...

TL;DR Add a feature to the Lua API that allows scripts to get a player's current inputs/keypresses , or to listen for something like an on_input_released event . What ? I've just run into a brick wall trying to implement a mod feature that involves holding down RMB, because currently Factorio does ...