How do i add an custom activity field to an email in civimail?
This doesn't solve anything, but a note for future development: contributions have the same situation where it's ambiguous which contribution you're talking about if you're emailing a contact.  However, there's a nice feature for sending thank-you letters (PDF or email) that has the contribution and contact tokens.  The difference is that this is accessed from the contribution search, so you're explicitly picking the contribution.

Something similar could be done for activities: if you do an activity search, it would be nice if emails generated from there allowed for including tokens referring to the activity.  Likewise, it would be nice to have a button from the activity saying "send an email regarding this activity".