[...]
Question is, what do we really want to have behind that function?
The delivery format is less important than actually providing help.
I guess that's set. But how would such a thing look like?
Well, since you are trying to get GP plugins to provide a ReST formatted readme why not make it a recommendation for a ReST formatted help which the GP build script can turn into an installed html file. So the plugin_help() just needs to call utils_open_browser().
Use the same css as Geany's manual and get a consistent look and feel.
As to content, first guess recommended layout:
How to use -------------- What it does, menu items, keybindings
Preferences --------------- blah
How to configure ---------------------- blah
The last is only needed if there is configuration beyond the preferences settings.
Cheers Lex