You can look for other console commands that do similar stuff, look in the code for cv_homing or cv_ringslinger and see how their "on_changes" work.
You don't need to go have the game editing the S_SKIN, because the S_SKIN is just a script that tells the game what powers to give the player, right? So all you need to do is look in the code for whatever thing you type in the S_SKIN to get whatever power, the text is linked to whatever boolean or value is used to turn that power on, so you can backtrack and look for what the game does if the power value/boolean is true or at a certain value.