The Army cares. I realize this is a little bit old, but thought I'd comment anyway.
I'd definitely ensure your command knows what they are signing the leave form for. Your unit can recall you from leave at anytime, so your true inability to respond will need to be known.
Your ability to remain a Soldier could be effected by the surgery, if things go sideways. I'd get approval in writing in case you need an LOD. Complications are rare of course, so I don't know for sure how all this would work out. However I would think if you volunteered to get sliced up and ended up with a disability, if the command approves it, it'd be covered just like a car accident (can't guarantee that's true), but if it was done on the sly you'd get shown the door (I'd guess UCMJ is uncommon, but in the realm of possibility).