More flexible tagging preset dialogs

[gaben]

In a new plugin, I'd like to override some core JOSM preset layout and input fields behaviour. Unfortunately, it's not possible so far.

Why? Because the plugin would duplicate part of the default tagging presets with some extras.

I'm okay with programming a preset for myself without touching JOSM core, but it isn't trivial either, so I'm open to suggestions. The closest thing I found is the roadsigns plugin which has some of the code I need (source available in josm svn repo, here).