I've been wanting to do somthing like this for a while, but I haven't really got into so far. tbone made an interesting system for mini-games called 'sims' on his
website.
It's not geared towards combat (the example is a somewhat violent dwarven sportsball thing, though), but the more-than-quick-contest / less-than-full-on-simulationist-combat are dang good and might be a good starting-off point.
In general I would follow the formula tbone gives: some rolls are static modifiers, some are mandatory, some are random challenges and some should be players' choice. In combat, I'd also add willingly expending resources (FP, energy reserves, powerstones, even potions etc.) to get another bonus on the final roll. Also if things actually get dicey you can narrate the opening stages and then pushing players into GURPS combat in medias res. Though that might work better with some groups than others.