I wrote to SmallCubed Support on this same subject back in September 2017. So this problem with MailHub has been going on forever. Here's what they wrote back to me. It would seem that all Anthony has to do to get things started at least is ask SmallCubed to have a look and consider taking over the code.
"We have had a few people approach us about adding MailHub like functionality to Mail Act-On, but the Mail plugin community is a small one and we try not to step on each others toes.
Last year we added DockStar like functionality to Mail Perspectives, but we didn't do it until the previous developer approached us and asked if we would consider it so that he could give his customers a place to go once he decided he couldn't justify supporting the product any more. We wouldn't consider adding Mailhub like features to our plugins unless we were approached by Dervish Software to do so, and even then it would only be if the extra work seemed as it if would be worth the investment.
Mail plugins are hard to maintain. Apple doesn't support them and the constant changes in Apple Mail means that you are constantly fixing code just to keep the plugins working. On top of that you need to find time to add new features that customers have requested. Taking on more functionality at the risk of falling behind on our existing plugins would not be doing our customers any favours.
For the moment you will need to be patient with Dervish Software and hope that they can find the time to update the plugin soon for High Sierra."