I like my Umlauts, but I usually enter them using "Option+u u". However, when I press "Option-u" in an Account Creation (or Edit) dialog, the input field loses its focus (it does work in "Description" fields when entering transactions). It also worked in the initial account creation dialogs. Here I have the workaround of entering the letter in some other application and copy'n'pasting it, and I don't need this dialog that often, but it would be nice if it worked anyway.
I've checked various default binding lists and can't find either opt+u or alt+u anywhere. I also don't see it in the Gnucash bindings at least not in my accelerator-map file.
I just found that option-o which should produce o-slash (ΓΈ) instead closes the dialog window.