Like Coops, I've camped around by the Cromlech boulders, arriving late and leaving early, but I get the impression that in the last year or so the laybys have become so popular in most mountain areas that I wouldn't bother now. I haven't heard of anyone getting in trouble but I think it'd just be much more pleasant walking up and out of the way a bit more, like Cwm Glas. Or take bivy gear along the horseshoe and drop down into Cwm Glas or another similar spot for a bivy en route.
There is a cheap and basic campsite just down the road in Nant Peris for £5 a night, but you can't book.
