Another way would be to add an option in the opam file. We can have something like external-build-systems: [ "rust" {">=1.36.0"} ] at the top level of the opam file. For each build system, you define a structure to define the dependencies. For Rust, it might be the workspace. In addition to that, the corresponding compiler version can be downloaded and added in $OPAM_SWITCH/bin (let’s not forget that multiple versions of a build system can be used for different projects, and it should not depend on a global installed version).
When downloading the opam dependencies, the external build system dependencies are also downloaded at the same time. For Cargo, you can use cargo fetch (see https://doc.rust-lang.org/cargo/commands/cargo-fetch.html). Of course, it is supposed external build systems allow this feature (it would be a condition sine qua non to be supported by opam). It should be possible to enforce only a list of URL’s like the cargo repository or some project hosting services like GitHub/GitLab. It does also require to trick the Cargo config to only check the sources in the opam switch! $CARGO_HOME can be used (see https://doc.rust-lang.org/cargo/guide/cargo-home.html). For info, the Rust sources will be downloaded in different directory depending on the dependency type (from a registry, from git, path, etc).
On the building side, as the dependencies are now on the user machine, in a specific directory, with cargo set up correctly, with the correct version, in the opam switch, the user can trick the build section in the opam file. It does also allow the user to use the external build systems in other sections like run-test!
It does look like option 2 with some additional ideas.