I imagine this isn't possible...but it would be good to just pull a selected random selection of mentions.
So for example, if a client wants a big topic pulled but for a small purpose so you could just run a 10,000 sample of mentions within the topic.
We are currently exploring a sampling option for Twitter realtime data. So instead of collecting 100% of tweets around a particular topic, you will be able to collect a percentage sample of the tweets available.
I will keep you posted on when this goes into development, at the moment this feature is on our backlog.