Yes, you can install a truss rod with a flat chanel, but if you cut it with a router, you could also use a bevel bit and put it in that way. and it will work with an angled head neck.
and with the carved top idea, it sounds very intresting, and it woulnt effect the tone much at all, but it would be VERYY hard to get it all to look right, because its hard to carve all of it so that it will look right with contrasting woods i would geuss.
but you can give it a go
ps plywood is bad because of the wood they use, its not good tone wood at all. if you use tone woods it wouldnt sound bad, just a little different cause of the glue, but not bad.