Sorry, I was unclear/incomplete in my explanation. The Visigoths got their alphabet from the Phoenicians (via Roman and Greek), with some futhorc runes thrown in to represent the elements of Germanic languages that Phoenician (a Semitic language) wasn't equipped to handle. Germanic tribes of course had their runes from well before that. This was the monk Ulfilas in about the 3rd C. CE.

Anyone happen to know offhand whether there's even an extended ascii code for the eth and the thorn?

[Edition]
One source is of at least passing interest online is a SIL/WBT site at http://www.jaars.org/museum/alphabet/