[LUGOS-SLO] Code for Bohorič alphabet?

[Tomaz Erjavec]

Dear all,
in the context of a historical corpus of Slovene I'd want to mark texts that are written in the Bohorič alphabet (http://en.wikipedia.org/wiki/Bohori%C4%8D_alphabet).
So far I've just been using @xml:lang="sl-boh" but I know this is sinful - but I'm not sure how it should be encoded.
First, I'm not sure if it even qualifies as a "script", as e.g. I can't find a script for old English which used the long s, but maybe because this only substitutes one character for another - with Bohorič  it's more complicated. 
Even taking it as a script (so I could write sl-Boho), http://www.tei-c.org/release/doc/tei-p5-doc/en/html/CH.html#CHSH does say that they should be taken from ISO 15924, http://unicode.org/iso15924/iso15924-codes.html and there is no Boho there; I also can't find an extension mechanism as there is with languages.
Any tips gratefully received.
Best,
Tomaž
-- 
Tomaž Erjavec, http://nl.ijs.si/et/
Dept. of Knowledge Technologies, Jožef Stefan Institute, Ljubljana