Thanks to advice from Jamie Bramble, I removed the X-Man jacket before beginning any major work.  I boiled-and-popped the X-Man head from the body.  Unfortunately, Batman's neck peg was nowhere near the same size as X-Man's.  So in order to maintain articulation, I had to perform minor surgery on Batman's head using a dremel (how many people can say that?).  But hey, it worked, and his head still moves.  For all this work, it must be obvious by now that articulation is pretty important to me.

Next, I started cutting and sanding the kneepads off.  I also sanded down some "stripes" on his arms, as well as some detailing around his neck and gloves.  Then, I created boot-cuffs with a rubberband, and using thin wire, I created a ridge on his arms for his jacket's sleeve.  To sculpt details on the helmet (including the fin, which I feared would be delicate and fragile), I used my free sample of Magic Sculp for the first time.  Not bad stuff at all.  It cured extremely solid, and it doesn't appear like it's going to break at all. 

And that's it.  This figure is extremely posable, even if he's somewhat twisted at the waist.  When displaying him at home, I keep him posed in the kneeling position shown in the second picture above.