I think you've got your wires crossed a little Neil.
LUA isnt the engine, its the scripting language, GRIME is the engine. With older games SCUMM isnt the engine, thats the scripting language SPUTM is the engine of which SCUMM, INSANE, CYST, IMUSE and FLEM are modules. EMI used the Jedi Knight2 Droid engine for graphics.
They could incorporate mouse control if they wanted to, they just dont want to (ignoring issues of how it would work).