In ancient German texts one used a small e on top of the a, o or u to denote an umlaut. (Compare to the second line here: http://de.wikipedia.org/wiki/Datei:Umlaut_forms.png\). It would be cool to have this style for umlauts available in Linux Libertine as distinct glyphs or via an openfont table. They are actually used for transcriptions of ancient texts. Poorly, I don't know whether they have any Unicode code points.