Nice work, but I still think, that it will not fulfill the planned purpose.
Only some geeks would use it, most people will be scared. I had people asking me, if I have no fear about electrical shocks, when using the digispark ;-)
Furthermore: I try to avoid attaching my contact information to keys, as that might be used to open my door. So I use some service cards, you can get usually from shopping companies, automobile clubs and other. You register with them, and they will be man in the middle for returning the keys.