It seems that the best place to start is getting the most you can out of contexts before starting a tagging system. I found this link to be very helpful:
http://www.evomend.net/en/what-not-gtd-context
He describes "the intersection problem" as mentioned with some solutions, too.
Already mentioned for this case are, I think, a couple of very good solutions -
1 - you need to answer "what is the tool that is absolutely necessary to the task?". In this case "Agenda: Boss" would likely work best with this,
2 - You could follow up with a separate task with a due date for later such as "Email boss if we haven't spoken about ..." with a context of email.