Take a look at Erlang's "parse transforms" which would allow you to implement some syntactic sugar. That's what is used to implement qlc, the query language for ETS/Mnesia - that one adds new semantics to list comprehensions, but you can modify any part of the syntax you want.
Also, Elixir supports macros and infix operator overloading - have you considered using it? If you know Erlang's stdlib and BEAM, switching to Elixir (and back) is almost painless. Not sure which monads you needed, but `with` macro is built-in, and it's a monadic bind for Option types (well, more or less). Adapting it for other monads shouldn't be hard.
For reference (for the "parse transform" approach in Erlang): https://github.com/rabbitmq/erlando - it doesn't look maintained, but it's probably still usable; otherwise, you might get some inspiration from the code :) This also (ab)uses list comprehension syntax:
I gotta say though that lack of infix custom operators for the monadic bind is a pain.