Well what a coincidence, up to this topic I had never heard of this cross Wales route and wierd!!
I was at a funeral yesterday, met a bloke, got talking and he had done both the Across Wales Walk organised by the West Birmingham Hostelling Group and The Rotary Across Wales Walk. He and his wife are more runners than walkers but the info was that the first goes E-W and the latter W-E. The former is more loose in terms of actual route only requiring participants to check in at checkpoints and the rotary walk, which starts at the Dovey estuary is well marked all the way, with a good breakfast thrown in at the first checkpoint.
The dates are shown on the two links already included in this topic. I share Mike Knipes sentiments on organised walks, but knowing much of the terrain I would not be too worried about finding rough camping spots, provided the rule of 'Pitch at dusk, break at Dawn' is on the menu.
Alfred's route taking in Newtown will miss an important feature IMO of routing off the hills east of the Severn valley, where there are two river crossings at Llandinam and Dolwen and is a big deviation of the direct line.
Passing through Llanidloes is the lesser of the deviations of the straight line objective of the route, but, though I am not a peak bagger, I do recognise where high points are important features within the context of a route, Bryn y Fan fits the bill in this respect and Trefeglas offers a great pub (for falling down potions) and village shop for picking up supplies (if it still in business). It is not too far off the line and links in with RoW network.
Over nighting near Llyn Ebyr, has attractions as the track up through Pen y Castel wood to the Motte and Bailey on the hill suggest an excellent spot for a rough camp.
There may also be a retail outlet in the Hafren Forrest visitors centre and also at the dam, where there is, I believe, the Red Kite feeding station.
This cross Wales line has the potential to be almost totally off-road and with high quality ways, it excites me and as Alfred says
Because theses challenge walks are all one day events they're obviously not concerned with accomodation or water etc. so I thought it might be fun to just make my own route
it is great fun to create your own LDP, I have done 2 cross Wales LDPs (to my own design) and they have taught me a lot about the access network of our country.