I don't think this would ever work at Disneyland. I love the concept of it, but the execution leaves a lot of room for improvement.
My mom visited Knott's Berry Farm on a Saturday and used the boarding pass on the rides that offered it. It turned into a game of "who to ask". There were two types of employees: some who were educated about the system and some who weren't. At most rides she would ask for the boarding pass, and they would then call to someone in a booth for the ride and get a time to put on the boarding pass. This is so that they do not have to print out boarding passes with every available time all day long (something they couldn't do). They then had to give you the actual time which they put on your pass. Each ride was a repeat of this. Then, she would get to some rides and ask about the boarding pass and they would say that they did not know what a boarding pass was. She would explain it, they would call to someone in a booth and then she would get the pass.
The positives of the system were that she was able to not stand in line, and it avoided the need to rent an ECV for the day (like she does at Disneyland). The other positive was that she got to "wait in line" like everyone else, so she was not bypassing the lines. The other positive for her, is that when it was a popular attraction she would get a bunch of boarding passes in a row and use them as fast passes essentially. She went to Boomerang when the wait was an hour and ten minutes and got a boarding pass, the boarding pass time was actually to return an hour and a half later, rather than the hour and ten minutes. She went to Jaguar and the wait was forty minutes, she got a boarding pass that said to return an hour later. She went to Ghostrider, the wait was 30 minutes, and the boarding pass had her return 40 minutes later. In the meantime, she used boarding passes from earlier in the day to knock out three rides back to back, then did three coasters back to back. They have no way to verify if you already have a boarding pass, so you can essentially create your own fast pass system.
There are a lot of reasons why I don't think this will work at Disneyland, one is the sheer number of people who use the GAC at Disneyland vs Knott's. They would need an extra employee at each ride that offered the boarding pass to hand them out, put times on them, check return times, etc. I actually think it would increase abuse of the system at Disneyland should they choose to initiate it because they can be used as fast passes essentially. Knott's still has a relatively low attendance compared to Disneyland (not even in the same ballpark) and they can have the ride operator do two different jobs, load the ride or unload it and issue the boarding passes.
I also think of the confusion at Disneyland when you are waiting for say Dumbo and the ride operator cannot even remember to unlock the gate until a guest yells "the gate" and I think that they would never be able to handle such a complicated system.
I love the fact that you wait as long or longer than everyone else in line, rather than get instant access, but I think Disneyland's system is tried and true for the number of guests that they have, and it's working, so I can't see any reason why they would change it. For people who think a GAC card at Disneyland is instant access, watch the disabled lines next time you are at Disneyland, chances are you will find yourself done with Small World or the Jungle Cruise before the person at the end of the line even gets half way through the line.