
Cliffs (vertical) are always two tiles wide, so Why not use on of it tiles (bottom one) to build underground inserter directly on it? It's just minor graphic change (just cut/mask half of it) and restrict possible directions (like with offshore pumps). Exactly same idea is valid for underground pipe.
Then, You can bend belt directly along the cliff side, with no space wasted.
Horizontal Cliffs are some harder (3 tile or 1 tile), but this also should be implementable. Diagonal cliffs maybe a problem...