As it was explained to me, the procedure is to ask the CM working on the platform.
Here was my experience 3 months ago:
We were the only ones in line for the monorail at Contemporary. As I had been previously instructed), I asked the CM on the platform if we could ride up front on the next train. He said we could, if it's available. After the train arrived and stopped, we saw a family exit the pilot's compartment. My excitement deflated, however, when I saw a father dart out of one of the monorail cars and run to the pilot, apparently asking if he and his family could ride up front. I couldn't hear, but the body language on both ends told me the pilot turned the man down. I then walked up to the pilot and asked if my family could ride up front. He was not a friendly guy at all. He curtly told me he's not permitted to do it unless the platform CM told us it was okay. I told him that I'd just asked that CM, a grandfatherly guy, and he'd said yes. The pilot reluctantly welcomed us aboard.
He was stone cold silent as we did the loop from Contemporary to MK, which was absolutely beautiful at night. When we got off at MK, however, he smiled and was friendly and gave the kids their co-pilots licenses and wished us a good night.