Unless you're talking of a different mod, mine's the only one I know of that has a pirate.
Anyway, I haven't done this as long or as deeply as everyone else, but I've trial-and-errored a few things together. Guts is pretty clunky and finicky, restricting what you can do to a relatively rigid scheme that allows some things but not others. I don't know much about how to incorporate randomness, but I do know that this page more or less says more or less what stats can be read by skills:
http://docs.runicgames.com/wiki/Static_StatsWhat's weird is that you have to turn on a "stat watcher" for these to work in your mod. It took me hours to figure this out once when I couldn't figure out why a mod wasn't working.
EXECUTE CHANCE or maybe EXECUTE CHANCE STAT is what you would want to read. I don't know how to use this in conjunction with randomness, though. You could easily have skills that scaled only off of skill level and execute chance, though. Someone else with more knowledge may be able to say more, but I've had trouble getting logic from different components to talk to each other and do their thing in the past.
If you had a skill that scaled off of skill level and execute chance, then I think it would also scale off of STR or FOC depending on whether the damage was physical or magical. As far as I know, there isn't a good way to have skills scale in an arbitrary multivariable function sort of way - you can pick one "stat watcher" stat, and then you get that times a number in a fixed random uniform range, times built-in damage multipliers (which I think are totally hard-coded, but I could be wrong). The way that your skill scales off of the watched stat could be non-linear, which you can do by constructing the graph of such a function in the Graphs editor.
The biggest issue I have with the Pirate class as it stands now, though, is that there isn't enough incentive to spend much time in melee, and I ran out of ideas for what to add. So it's mostly a shooter, but Cutlass Sweep is decent, and Pirate Handshake is a nice dirty trick to have available from time to time. I've also wondered if the class would have enough defenses even if it had more melee skills anyway, and if not, what sort of tweak might fix help. An extra effect on Skull Mug (so far, it's a regeneration buff granted by kills) could help, but my two custom classes have a fair number of mandatory skills as is, where vanilla classes don't seem to put as much weight on passive skills.