I\'m a afraid I can\'t answer your question, but I\'m curious, are you doing handwriting in Icelandic or Old English, that you need the thorn and the edh?
The customize keyboard plugin, which can either display a pick list for characters or convert arbitrary key sequences to other characters (e.g., \"c\" + \";\" =] \"ç\", or as I use it Shift+\"(\" =] \"[\"), can add the thorn; set up the customize keyboard, then select the thorn from the Unicode plugin.
I couldn\'t find the edh in the unicode list, but presumably if you know the unicode byte sequence for it and have font that displays it, you can add it to the customize keyboard plugin as well.
Custom keyboard is well worth having.