Yeah, what you asking for is exactly what tool lints are for. This is already an accepted feature and implemented in the compiler. Currently the tool lint feature can only be used (by implementer) by Clippy and soon rustdoc, because of a a hard coded whitelist. This was also the case for tool attributes when those were first implemented and then was removed once the first non-official tool wanted to use tool attributes.
If you want to use tool lints attributes, the only thing that currently blocks you is that hard coded whitelist, which causes the unknown tool error. The only reason why this whitelist is still there is that no one complained about it. If you're willing to put some work into this (mostly writing tests and making sure existing tools don't break), I'm supporting removing this whitelist and open up tool lints for everyone.