I'd like to think that most people who create a plugin based on an answer here already go back to their answer to add a link. I certainly do.
As for the box layout, it is the proverbial bikeshed. I have so far just added a simple link to my answers, but if people want a consistent box then that's fine. Either way, editing an answer to add a link to a plugin which was created from the question should be fine in most cases (especially if the author of the plugin is the same as the author of the question).
As for maintaining a list of plugins, good idea! I have just started such a list here.
I included my own plugins and the ones in this questions except for vim-ditto, since it wasn't really clear to me from which question this was inspired and it was phrased as "help with" − to be clear, I don't think we need to list plugins that were helped out by an answer on this site, just plugins that were inspired by a question on this site.
Feel free to expand this list.
From the comments:
I think the idea isn't bad but there is a risk of abuse: it could lead plugin creator to ask (not relevant) questions in order to advertise their plugin
The important point is to 1) have clear guidelines about that
The "risk for abuse" is already present. Linking to plugins is not only allowed, it is encouraged. It's just that answers that offer only "try this plugin" without any further explanation are considered undesired.
If you can ask an on-topic question and write a decent answer to that, then by all means go ahead. I couldn't care less if you did it only to promote your plugin, if the content is good, then the content is good.
I don't think we need any new guidelines for this.