I've wanted 1 sided walls for some time. The only work around I can think of is inefficient. Basically it'd be a setup of command blocks and spawn blocks.
there's a simpler, albeit worse looking alternative.
have your wall built with 16x16 tiles, and on the left of it, place a repeating command block with something like:
if key = right
character x + 48
maybe with some experimentation you could make that look good, but it's not a realistic replacemant. that'd make a fine addition to the game
