I've always thought Joss as a nn for Jocelyn. Even if you click on it it says Joss is a short form of Jocelyn. I'd go with your suggestions of Joss and Jossy / Jossie or even Miss Claire's spelling Jocie as nn for Jocelyn. Jojo could be but it requires changing the pronounciation of the 'o'. Lyn could be too. You caould also try asking this on the name opinions board for more responses.
How is it that Beth is an "official" nn of Elizabeth? What about Beth for Bethany? How exactly do you have official nn? Is it a case of everyone knows that this nn is for this name?