Maybe you like to have a look at commander-plugin which already might is a good starting point.
Possibly, and thanks for the pointer, but I'm not really aiming to produce a full plugin. Lua scripts are much quicker and easier :). One file, no compilation, just save to a directory and run them.
Personally I doubt the lua-plugin is a good base for such a feature as it's nearly unmaintained since some time.
Hmm...thanks for the tip. As far as I can tell, though, the Lua plugin still works fine, whether it's maintained or not. I've produced several scripts in it.
Either way, do you think that a vim overlay would be useful (to some)?