by Davo Smith.
You could try my Navbuttons plugin ( https://moodle.org/plugins/view.php?plugin=block_navbuttons ), but that does required a (small) change to core code to work (it might be possible to rewrite it now, so that the core modification is not needed in up to date versions of Moodle, but I've not had time to look at that).
It does not do exactly what you want, but could be combined with conditional availability, to get a bit closer to that.