Looked into how that board did it a little.
HP seems to be a measure of post count, and your current HP is a measure of how active you actually are (related to your posts per day).
Energy is how long they've been on the forums, and their post count during that time.
Not sure how the leveling up process works though, I was just glancing at it.
This is the FAQ that deals with the system on gwonline, if anyone is interested.
I'm wary of trying to incorporate it with Darkstorm or a similar RP game, since that would end up excluding the majority of our members from the scheme. It's still a possibility, but I think having a profile field that is only going to apply to a small fraction of the active members might be a little silly.