Adrian, I've not got anything to do with this particular meet, but you should be fine. Meets like this one organised on public OM threads are generally open to all, and the link in the first post to the climbing barn states that it sleeps up to 30.
I expect Maria will show up to confirm that at some point, but if you're up for it then add your name to the list.
Another nice report, Geoff, good to see the v-g list growing again this year
We realised we'd virtually never used Bethesda as a starting point, and earlier this summer did a long day walk that shared your route from Llewellyn onwards. We parked in the A5 layby outside the caravan park just south of Bethesda and took the footpath up through the forest to reach the Afon Berthen. After that it was trackless but straightforward (and steep!) up onto Pen yr Ole Wen. This route avoids any scrambling - it's maybe not as spectacular as Cwm Caseg, but it does give great views across the valley to the Carnedd y Filiast slabs. (There'd be some scope to camp by the upper Berthen, or on the ridge if you carry water up.)
Our return was like yours except that we continued more directly SW towards Gerlan, and encountered difficulty with the final paths and fences to get back to tarmac - I reckon we'd try your northern variant another time.
Whenever we've camped high on the Carneddau we've taken water from the pools like that one. It's been fine filtered through a Drinksafe or Sawyer. Did you filter or get your water from some other source?