Hi Tiny, I know this isn't exactly what you're asking but heres a thought for you. I've been using TwitterMoneyBot for mass follow (free edition, haven't paid for it yet). It's got a good search function and you can search and follow in the free version (no unfollow, sadly). I search followback and 'teamfollowback' - delete the users with no avatar, delete the users with links as their tweets and then delete duplicates - you get about 50-60% followback rate (which in my experience is quite good).
Another tip - and I've seen others mention this so I know it's not original) - have a sacrificial account and a real account. I have my sacrificial account follow 100-200 people off the followback search. Then I have my real account follow the followers of the sacrificial account. This way my real account only follows people who've already proven they will reliably followback. With this method I get a 90% followback rate on my real account. Then I do some batch unfollowing on the sacrifical account and keep doing it until the sacrificial account gets suspended. Then I take a new sacrificial account (preferably one I've aged for a few months and tweeted with to make it safer) and repeat.
Again, not an answer to your actual question but perhaps helpful?