[rhl]

Well, total reach might be tough to pull off, but it would be definitely feasible to poll the Twitter API every hour and count the tweets with that hashtag that appeared in that time span.

To justify cost to brands it might be easier/more 'sellable' to insist on actual, measurable engagement (links clicked, tweets retweeted, tweets favorited etc.) around their own tweets. I'd worry about unrelated tweets (i.e. sharing the same keyword but about something else altogether), or either bad publicity being accounted for.