The celebrated Bosworth-Toller dictionary of Anglo-Saxon uses a curious diacritic to mark long vowels. It may be described as a long shallow acute with a small down-tick at the right. It contrasts with an acute (quite steep in this typeface) used to mark accented short vowels. Both can be seen in the fifth line of the scan at http://lexicon.ff.cuni.cz/png/oe_bosworthtoller/b0002.png
What is its appropriate Unicode representation? As a lumper, I would use a macron, but I wonder what a splitter would say.
