Oh that's very nice.
One follow up: can I import my version 0 address into Multibit HD to do some signing with it? Or is HD only for HD?
[1] https://bitcointalksearch.org/topic/m.10622773
Thanks for the link, Shorena, that seems to answer my question definitively. Good for future HD users that they can sign and verify more conveniently but seems like for version 0 addresses that I'm going to need to use another client for my signing.
You could create a migration message as you would when switching PGP keys. Create a single message stating your name, date, the old address, the new address and that all further message will be signed with the new instead of the old address. Sign the message with both adresses and spread to those that need to know.
I don't see how this is relevant to outputting signatures to a file, which can apparantely be done in new Multibit HD but not in old Multibit, since new multibit can't be used with version 0 addresses, I think it's clear that I have to use another client for signing.
Thanks I guess for reminding me why signing is useful :/