Ketamine, maybe? It is a paralytic. This would be better with assisted suicide though, as it could cause paralysis that could prevent taking the next steps in the process. Maybe consuming it orally, with a beverage would slow the absorption rate enough. But, again, it would have to be timed very carefully to be useful and practical.
Another thought that comes to mind is that a lot of the methods that seem to be of the highest esteem in this forum also seem to be effective enough that they might also be paralytic due to their heavily sedative qualities: N, inert gas, and SN. This is all just educated conjecture though. Feedback from smarter folk than I is most welcome to further my musings.