I know nothing about bucklescript or melange so I don’t know if it’s enough, but the following command will create a switch named melange from a git reference:
% opam update
% opam compiler create 'anmonteiro/ocaml:75f22c872451c66a4c4aadc43abee55697268a57' --switch melange
...
% ocaml -version
The OCaml toplevel, version 4.12.0+BS