I haven't really been deep into tabletop RPG-ing for about 20 years, but I'd also believe that's the place to look for a good, explicitly open-source system. I think designing and implementing a generic/basic rule system isn't so hard, though, unless you're going down the simulationist road or doing something quite special (in which case you need to roll your own, anyway). Squirm was loosely based on White Wolf's Ars Magica and World of Darkness series, rolling for successes against a set difficulty. The system I'm using for LoSt is a bit more abstract, and not quite ironed out yet, but loosely based on the roll of a six-sided die.
When I was playing RPGs with my friends back in the days, the rules were never that important, because what mattered was coming together to tell a fun/good story. A tactical RL should probably have a system that's more tailored for the specific needs of that RL, though, lest it feel hopelessly "next-gen" (let's face it, next-gen RLs are just sooo 90-ies).
As always,
Minotauros