It's not just that it "dovetails" with it. It's that, ideally, it would answer most of the big objections.
@kornel's big objection was that finding C libraries is hard, and all that hardness needs to be concentrated somewhere (in Rust, that hardness is the build.rs file). But if the Rust package is being built by Meson, then it should hand the responsibility for finding it over to the Meson dependency checker, and the weirdness of finding openssl is now FindOpenssl.cmake's problem. The openssl-sys package might even continue to use build.rs if you were just using standalone cargo, only employing the declarative discovery system if the project-global "stop doing fancy stuff" flag were turned on.