this doesn't work:

let
  applyGetter = set: getter: builtins.addErrorContext "hi" getter set;
in
applyGetter {
  foo = 1;
} (set: set.bar)

but this does:

let
  applyGetter = set: getter: builtins.addErrorContext "hi" (getter set);
in
applyGetter {
  foo = 1;
} (set: set.bar)

is there any way to add error context for attempting to access an attribute that doesn't exist?

Yes:

nix-repl> builtins.addErrorContext "hi" {}.foo
error:
       … hi

       error: attribute 'foo' missing
       at «string»:1:31:
            1| builtins.addErrorContext "hi" {}.foo
             |                               ^

Does this answer the question?

we want to only show the attrNames on an error

nix-repl> printOnErr = set: name: builtins.addErrorContext "${name} not found. Available are: ${toString (builtins.attrNames set)}" set.${name}

nix-repl> printOnErr { foo = null; bar = null; } "hello"
error:
       … hello not found. Available are: bar foo

       error: attribute 'hello' missing
       at «string»:1:123:
            1| printOnErr = set: name: builtins.addErrorContext "${name} not found. Available are: ${toString (builtins.attrNames set)}" set.${name}
             |                                                                                                                           ^