In order for bracelets to "sell out," the park would have to be at capacity. That's highly unlikely. Every single person in every single resort would have to decide to come to the park for EMH, and even then I'm not sure you would reach full capacity.
We were there over Easter and while the parks were *VERY* crowded during the day, we had no problem with EMH in the evening. We stayed for EMH on the 23rd when MK was open 'till 3:00 AM. Our kids actually made it 'till 2:30! (We did let them take naps) It was GREAT! We got to see Philharmagic, IASW, walked right onto Buzz Lightyear, did Dumbo with maybe a 10-15 minute wait, went on the teacups 2 times in a row and walked right onto the Speedway. There was still a long wait for Space Mountain and no Fastpass operating on that one.
When we went to Epcot for the evening EMH, there was a Fastpass for TestTrack, but the line was quite long before they even "opened" the machine. Our ride time was over an hour away from the time we were there, but it gave us time to see the Journey Into the Imagination ride and the Honey I Shrunk the Audience before it was time for TestTrack.