Just spitballing, but you could have composition eat the name of an input to the outer function: Given f(x,y) and g(z), write

let q = (f .y g)(z=a) then q(y=b) = f(x=g(z=a), y=b)

let q = (f .x g)(z=a) then q(z=a) = f(y=g(z=a), x=b)

Aesthetically it would be kinda like the notation for fiber product.