Legally you cannot ask for documentation of a person's disability.
Yes.. but you can require a note from a doctor without stating the actual disability. Six Flags requires a doctor's note and I think more parks should move to this system.
Your doctor should include the following information:
Address of the doctor's practice (Printed stationary is OK as long as it includes the Doctor's medical ID number)
Phone number of doctor's practice (Printed stationary is OK as long as it includes the Doctor's medical ID number)
Name of person requesting the Attraction Access Pass
Statement indicating the guest has a disability or other qualifying impairment under the Americans with Disabilities Act (ADA) and/or applicable state law that prevents the guest from waiting in a standard queue.
Valid time period of disability (permanent or, if temporary, the valid time period the Pass is needed for)
The note must NOT describe or indicate the nature of the disability
Its also not a skip the line pass, it's a "not wait in the physical line" pass...
At BGT and SeaWorld Parks you get a time to return after waiting in a virtual queue. Same with Cedar Fair.