To contribute to Plack projects, simply fork a repository on github, push your changes to a topic branch and open a pull request. It's likely that personal emails to the authors and maintainers would be ignored.
The likelihood of your contributions getting applied would increase if:
No response to the issue or pull request would probably mean that it's not interesting or we're not sure if the proposed change has any positive value. Try justifying the rationale, or get +1 from the community to increase the chance of being looked at, by getting the issue more interesting.
Similarly, if you find an issue that gets stuck not applied in the issue tracker, and are experiencing the same/similar problem, get your voices in by simply upvoting it, or by describing how that affects your application.
If you want a quick feedback and/or would like to discuss when you're not sure if your proposal is a good idea, use the IRC channel #plack on irc.perl.org.