Assuming you have installed a custom compiler version, e.g. 4.08.0+foo, to
your PATH and want to use it in opam without recompilation, you can use this
package to treat it as a system compiler:
opam pin ocaml-system.$(opam var sys-ocaml-version) git+https://github.com/AltGr/ocaml-system
The trick is that this allows you to specify the custom version of your compiler, which is normally rejected by the ocaml-system package from the repository.
The package definition here is similar, except that it refers to its version
dynamically in the available: field.