I say to pitch the idea or 'outline' first. Have someone (on the payroll) responsible for sourcing opportunities, then ask your bloggers to pitch ideas. (For consistency's sake - consider bringing them together at a daily or weekly meeting?)
From there, pitch the ideas to the brands or agencies and paid the bloggers per post to flush out their ideas.
Pay-per-post is only successful / economical when the strategy is already developed.
You can't control whether someone else wants to have a conversation.
I've been blogging for almost 6 years, now with 40,000 RSS subscribers. I can tell you I have 0% ability to predict which post will be popular, causing lots of retweets and comments, and which are duds. Not time of year, not topic, not how excited I am with the piece, not length.
What that means, to me, is that you write a lot of good stuff and the conversations increase overall, but post-to-post it's necessarily unknowable.
That's OK, that's just the medium.
That's how I'd pitch it.
I've consulted with some of the largest blog advertising and online advertising companies in the world. Companies like IZEA (PayPerPost and others), MediaWhiz, etc. And I currently provide a robust Blogger Outreach service to many other large digital search agencies.
I'm not sure that I understand your question. But I think that you're asking how to get bloggers to talk about a client (or PR agency's client) without having it be a paid conversation?
This is tough if all of the bloggers in your network are used to getting paid. Chances are they really only run the blogs for the ad revenue, and not for an audience. That makes them a poor fit for PR agencies.
PR agencies contact writers and editors and pitch them ideas and story angles around interesting things their clients are doing. So you need to develop good creative ideas. One example is that at a previous agency I ran the CEO offered a gas subsidy program for employees who lived far away from the office and commuted every day when gas prices went really high at one point a while ago. So it was a BIG story in the news. Well we pitched our story to local media and within the week had them filming the story in our office!
If I did not answer correctly or if you would like more consulting I am open for appointments.