uff.. too much discussion here
.. anyway, for a bot, I still think the best is ask the user to send a command to the bot to enable (or disable if it's enabled by default) plus features
or the bot may send Ping? [msgplus] to see it the contact has plus. That's easy, will work with everyone (with plus 2.50+) (no need to upgrade to 3.0) and don't imply changing the nick, cheating with the protocol or storing per each user who used the bot if s/he has or not plus installed.