While it might be cool for it to attempt each ruby version in order (aka. mimic ASDF parsing)

I agree, that doesn't seem worth the complexity, at least not without a good use case.

or to treat multiple .tool-versions entries as a matrix, that's way beyond what I need and am comfortable tackling as a first PR.

Regarding treating as a matrix that's impossible, AFAIK an action cannot influence the matrix (at least in the same job).