This feature sounds like it would only be useful to players, not modders.
If a moddable research cost multiplier is added, then it should be stand-alone setting, not bound to game difficulty settings. That's because a player may not necessarily want a shorter+easier game or a longer+harder game. They could also have a shorter+harder or a longer+easier game.
You do mention another interesting concept tho - Game Length. I wonder if a player-adjustable Game Length setting has already been discussed in the past.