If you put it in the walls you have to do all the drops now or cut into the wall later and then patch the wall. What's wrong with having the pipe exposed? Any of the soft tubing choices will have undulations in the "straight" sections but you should be able to just use a steeper pitch to get a decent amount of self-draining without resorting to putting the tubing inside conduit. When you open a drain valve the blast should take care of any small puddles in the tubing
. Whatever you use, definitely use an upward pointing tee for each drop. We had to run new airlines in the shops at my last job and at first they just used downward pointing tees for each drop. Pretty soon they had water dripping out of the air ratchets and other tools when someone forgot to drain the compressor tank
. Rotated the drop tees to point upward and put in an elbow to a short horizontal run to another elbow down to the existing vertical drop pipe and no more water. I know you must have lots of reference texts and/or programs but one I've found really useful for small stuff is Engineering Power Tools from www.pwr-tools.com
. There's a freeware version that is pretty handy with no ads or other annoyances, and the full version is only $50 and well worth it, IMHO. Does lots of different calculations including moment of inertia, beam deflection for lots of geometries, plate deflection (handy for plenum wall and intercooler tank thicknesses when you are running boost), and tables of data and other calculations. Anyway, it has a table of pressure drop vs. air flow in pipes for 100' runs that shows 4.8 psi for 3/8" pipe and 1.4 psi for 1/2" pipe at 20 cfm, so I'd definitely not go smaller than 3/8" pipe or tubing, and 1/2" would be plenty. You won't run 20 cfm continuously but you don't want to limit the initial blast from a full tank of air with a blow gun or to seat a tire on a rim.