First of all, while we have talked about setting up a "Rust foundation" to allow donations to support Rust, one of the properties almost everyone is agreed on about such a foundation is that it should not directly pay developers. (It might potentially cover things like travel costs and conferences, but not developer salaries.) Down that path lies a great deal of strife between volunteers and paid developers, and potentially a net reduction in people working on Rust.
Second, any Rust foundation we set up would specifically want to avoid a "pay for play" structure, especially for things like "core team seats". Yikes. And while advertising of sponsors is fine, the only thing that would get a company/developer described as being a "co-maintainer of Rust" is actually doing the work to be a co-maintainer of Rust.
For the rest of this post, I want to pick out a couple of ideas from your list that I think do have some potential, if carefully separated from any the above issues.
The concept of paying to include private projects in crater runs is an interesting one, though then that would get into issues of "who gets to see the details of such code". I could imagine ways to solve that problem (starting with a simple boolean "something broke here" and a notification to the maintainers of the private repository), and it's an interesting one, but we'd want to firmly separate it from the rest of what's mentioned here.
For such things, I'd suggest talking to companies like Ferrous Systems. I could also imagine having a way for individual Rust developers who do consulting to advertise themselves as such.
These are certainly reasonable ideas.