What you are asking about is called a Guest Assistance Card (we abbreviate it as GAC on the DIS Boards) and they do have GACs at WDW. You can request one at Guest Services in any of the parks. For invisible disabilities, many people do bring a doctor's note indicating what problems there will be with waiting in line.
Things are a little different at WDW than at DL, because DL was built so long ago that accessiblity or people with conditions like autism were not considered. So, they do a lot more special wheelchair entrances at DL, since that's the only way they can do it. The Special Assistance Pass at DL often allows people with conditions like autism to use the separate wheelchair entrance.
Most of the rides at WDW don't have a special wheelchair entrance - people in wheelchairs or with mobility disabilities usually wait in the regular line.
There are different messages on the GAC, depending on the needs of the person it is issued to. There are not any GACs that will allow you to go to the front of the line, but for people who just can't wait in line with everyone else, they do have other alternatives, like waiting somewhere else that is more appropriate.
If you follow the link in my signature to the disABILITIES Board, you will find a lot more information.
Don't worry, WDW doesn't want anyone with autism havinga meltdown in line and they will have some help for you.