I'd lay out the general story and player requirements (e.g. "scenario might contain some hacking, some long-range engagements, some driving" and etcetera) and then use an online dice-roller to determine outcomes (so you'd say "I'm going to take a perception test on that Troll", I'd call it perception+4 then auto-roll a D20+4 to determine how it turned out). God help you if you're messing with that troll and roll snake-eyes, it won't end up pretty.
I'd be keeping track and either myself or the players would update HP and so on with each new post.
With that said, there are some forum specific systems that are quite easy to use. I'll be doing some research (particularly over at the SA forums) as I'm really keen.
I definitely have a scenario in mind, once I can confirm the ruleset I'll put up a recruitment thread.
I think it could be really good fun!
Note: this is not a story-RPG, it will be about dice rolls and stats. There will be some story exposition (like "A shot pings past your face" or "You round the corner and three shady dudes in trenchcoats are doing unspeakable things to an ATM") merely as scene-setters and the players will really be just calling out their chosen actions... but nothing even remotely approaching the feotid depths of our NWN RP forum