A parameter to consider while trying to measure effectiveness of an
outreach session is the time constraints of the person doing the
outreach. If working folks are involved, it would be difficult to get
this sort of feedback. Things get easier, though, when size of the
group with which outreach is done is less than 5. Perhaps doing
micro-outreach and relying on a domino effect would be more effective
in measuring effectiveness of outreach. Just a theory.

Great suggestion Pradeep. I agree, volunteer time is limited and is extremely valuable. Hence every minute invested by the community in outreach should have maximum possible impact. If greater effectiveness means limiting the number of participants to a number as small as 5 or 10 then we must do that. I'd include this point in the handbook or else fell free to add it yourself.

If we have enough community members then may be we should think of ways of dividing this group of 40 in 4 different groups and individual community members leading each group.
But the question is, what do we do when we don't have enough community members in that particular region. In such situations we'll have to conduct outreach for larger number of participants and find more efficient ways of following up with the participants post the session. Some of the ways could be:

Towards the end of the session ask the participants to select an article that they all would like to collaboratively edit during the next one week. Tell them that you've created a Google group for them where they could post messages, queries, comments etc. Keep a close watch on this newly created Google group and guide and help them by answering their queries promptly. Pretty much like what Ashwin is trying do with the GA master class.
Towards the end of the session ask the participant if any of them would like to become representatives of this group. You should maintain direct contact with these volunteers (these could be 1-2) and if possible ask them to organize follow up academy session where participants could get their doubts cleared or ask them to invite all the participants to wiki meet ups etc. Basically, these volunteers could serve as your single point of contact for the entire group making your task of following up much easier.