Oudsaksisch
I stumbled over an Old Saxon Incubator and they are using two signs for long vowels on some pages. I haven't the foggiest what each is supposed but I thought that might be of interest to your work. http://incubator.wikimedia.org/wiki/Wp/osx/Karl_the_Grôt
As far as I'm aware, one set of vowels is the 'originally long' vowels, which derive from Proto-Germanic ā, ē, ī, ō and ū, as well as au and ai. The other consists of short vowels that were lengthened. This process did indeed happen in Saxon, and modern Low German retains this distinction. It also happened in Dutch and High German but they did not retain a distinction between them. I'm not quite sure whether there is any evidence that the lengthening already happened in Old Saxon or whether it is a Middle Low German change. And that's made more difficult because there is no written distinction in writing, so the chronology can only be inferred. In Dutch at least, one telling sign is that short i lengthened to e, so there is at least some evidence in the spelling in this case, but I don't know about Low German.
No, it's not lengthened ones. They use ô for *au consistently. (Since /ō/ corresponds to later Saxon /ou/ and contemporary Dutch/German /uo/.) So maybe the circumflex means 'no diphthong'. I cannot speak for 'ê', though. But I consider it good practice, since a proper Old Saxon declaration makes it easer to reconstruct whether later people said /hø:ved/, /hœ:ved/ or /hœʏved/.