Have you looked into Paramo or Furtech WPB systems? It's *somewhat* similar to your idea. The big difference is that the "fleece" they use is thinner and "directional" to create a capillary action. Basically, it's a mid weight polyester shirt with hood that is fuzzy/fleeced on one side and smoothly woven on the other side. The fuzzy/fleeced part goes on the outside (these are the smaller fibers), and the smoothly woven part (the larger fibers) goes on the inside towards your skin (they recommend wearing a baselayer though). They are expensive as heck, but you can easily replicate it by combining a good wind jacket, finding a one sided fleeced/brushed polyester shirt (preferably with a hood), and then DWR treating the shirt.
If you use a windjacket that has lot's of seams on the shoulder area like my Houdini does, then i recommend you seam sealing those seams first.
It's only ideal for temps below 50, and i might say more so below 45. It's not also ideal on a long thru hike because the DWR treatment on the inner "pump liner" (the directional polyester fleece) can degrade. Pack strap areas are notorious for getting wet during a heavier or sustained rain (however, many say it dries out very fast).
It's THE most breathable WPB system out there, but like all WPB systems it's not perfect and has it's pros and cons. If the inner liner does get wet, it dries out really fast. One could also use one of those polypropylene baselayers that is brushed on one side as a "pump liner", and not have to worry about the DWR treatment as much (though you definitely want to periodically wash it well). But polypropylene is both rather warm and stinky stuff. However, wearing a merino or alpaca to synthetic blend baselayer would help with the stink part.
As an alternative, you may also want to look into ponchos that use a WPB material. This will be the next most breathable system, and definitely cooler than the Paramo type. If you're into MYOG: A partial eVent Poncho would be sublime as far as breathability. One could do about one half (a little over) in eVent and about one half (a little under) in silnylon. The eVent part would go over your front, and the silnylon would be in the back and go over your bag. This would both bring the price and weight down, cause your pack doesn't really need "breathability" like you and your front does. Hmm... (i just thought of this, and really like the idea, now to source some lightweight eVent material).