In reply to DaveHK:
There are a few places on or very near to the route - I've left my car over night (or slept in it) at most of them without any problems.
Capel has the carpark behind Joe Browns store directly on the route, as well as the Siabod caffi and Tyn y coed carparks mentioned above which are 600m/1200m off the route E on the A5, or the layby between the two halves of llynau mymbyr 600m W.
Ogwen has several large lay-bys along the lake that are regularly used for overnight stays.
Llanberis is more awkward, but a more likely spot would be the Bus Stop Quarry area in Dinorwic - an easy 500m off the route as it passes through the slate quarries.
There's a secluded carpark in the forest, about 600m South of where the route crosses the A4085 SW of Snowdon (Pont Cae'r gors).
The carpark marked in the map at Nantmor is a National Trust one and I don't know but it may be more of an issue for long/over night stays, especially if not a NT member.
There is also a small carpark in Croesor (I haven't been to that one).