It seems to me that given 5 years notice, your company should be able to figure out a way to accommodate you. I have no idea what kind of company you work for, nor what kind of a role you play, but what company is so poor at planning that they cannot pull this off.
I gave my company two years advance notice of my trip. I didn't really "ask". I just told them what I "needed". They didn't exactly say "yes" or "no" either, but just acknowledged my statement. I didn't ask for a "yes" or "no" answer either. Then I just keep reminding them at every opportunity that this is coming up. And I also pretty-much told everybody in the company that I was doing this, and posted my detailed plans on my internal web site. After enough repetitions, it just seems to everyone that this is a "done deal", even though it was never formally approved. Now, it's just generally accepted as a "fact".