I don't think the umlaut makes that much sense. It is rarely used in English and it wouldn't be used over an e in German. Plus it will probably be a pain to explain, type, etc.
Wikipedia article:
http://en.wikipedia.org/wiki/Umlaut_%28diacritic%29
It does say in there that
"In English
The diaeresis mark has also been occasionally applied to English words of Latin origin (as in coöperate, reënact), and more rarely in native English words (noöne), but this usage had become extremely rare by the 1940s. The New Yorker, The Economist and MIT's Technology Review can be noted as some of the few publications that spell coöperate with a diaeresis.
Its use in English today, apart from words borrowed from other languages, is mostly limited to certain names, such as the surname Brontë and the given names Chloë & Zoë. It is probably most common in words that don't have an obvious divider at the diaeresis point (the diaeresis cannot be replaced by a preceding hyphen), such as naïve."
So it wouldn't be totally inappropriate but my first thought was no umlaut.