When we were at Disney, a few years ago, we had to rent a wheelchair for my youngest DS. We did not have a GAC, nor did we expect any special treatment, but at every ride we approached the CMs were wanting to wave us to the front of the line. I know this does not really answer the OP's question, but I just wanted to comment on Disney's approach to someone with perceived disabilities. Thankfully it was just a minor thing with my son, but I saw just how great Disney can be with disabled guests and their families.
This is pretty much what we experienced in Dec. 2010. It was rampant.
ECV, wheelchair, cane, knee brace, you name it.....anything remotely visible and you and six of your nearest and dearest went right into the fastpass line, never mind that none of you had a fastpass. It was especially popular at TSM. We went exactly one year earlier and I don't remember seeing this happen, even though I'm sure it did occasionally.
But this time we saw it in every park, all 10 days.
Often. It was like an epidemic. So when someone protests that a GAC or a disabilty does NOT get a person faster access, I have strongly disagree. Maybe it isn't SUPPOSED to, but it can. My theory is that either the CMs don't understand what a GAC allows or they don't want to get into a disagreement with a guest who expects to go right in, so they just let them in. As with everything else, some people follow the rules and some feel entitled to special treatment. (I'm not talking about you....if they waved you to the front, that wasn't your call.) But the ones who DO feel entitled also tend to be the most vocal, and I don't imagine CMs want to hash it out with them if just quietly letting them go via the fastpass line makes it all go away.
My fear is that as word gets out and as more people start to see what is happening, those with GACs who have been following the rules will look at the "instant fastpassers" and ask themselves, "Why do THEY get to ride _____multiple times while I have to wait in line? What's up with that? They're no worse off than I am." And I can't blame them. This is getting to be so common that it's bound to get ugly and the number of people taking advantage will no doubt multiply rapidly, as already seems to be happening.
As to the "pieces of paper with writing on them," I may be able to shed light on that. DD and I were in line for Dumbo. We were ready to board the last Dumbo available and realized that we were not the only people wanting to get in said Dumbo.

It was between us (DD and I) and a mom with 2 very small boys who looked like they were about to drop. Even though DD and I had gotten to the Dumbo first, I told her to take it because her children were younger.
When the CMs saw us walking back to the holding pen, they told us to get on a Dumbo, I told them there WAS no Dumbo for us, and explained that I had let the other family have the only available Dumbo since the kids were younger. Let me just say that they started number crunching. Apparently, Dumbo is a tightly run ship and the numbers must be precise. That baby was not flying until they solved the mystery. Turns out someone had told them that they would "all 4 sit in one Dumbo" but when they saw the miniscule size of the thing compared to their rear ends, they decided to take TWO Dumbos instead of one. Yes, they bogarted our Dumbo.
We couldn't just ride the next time, because they were already loaded for at least the next two times. So they got out two slips of paper and scribbled a few things on it (maybe their name, the number of people in my party, the date....) and gave the two pieces of paper to me. Little xeroxed pieces of paper about the size of a fastpass. Each one was a sort of like a fastpass for the 3 people in our group, for any ride in MK, except that the paper was good even for rides that didn't have fastpasses. If we wanted to use it for something like Dumbo that had no fastpass, we could just show it to a CM and they'd take us to the front of the line. It was to make up for getting diddled out of riding Dumbo when it was our turn and also for giving up our chance to ride so someone else could ride instead. That's the story behind the pieces of paper.