In reply to Chip Bennett.

It’s easy to say “you can just donate code” – but where and how do you do that?

The plugins Trac is probably the best place to submit patches if the plugin author did not mention any GitHub repository in the readme. It's far from perfect, but it's not more difficult than submitting patches to WordPress Core IMO. And this trac recently got the same redesign treatment than core.trac, so it looks good now! :)

I would also suggest posting a link to the Trac ticket in a new forum thread to let the plugin author know about your patch, just in case.