The problem with going after rentals is that it's really hard to build a value proposition. Why would a landlord want to pay a Realtor 1 months rent to find a tenant when it's probably easier and FREE to do so in Craiglist since there is such a huge pool of potential tenants looking there. Just look at MLS and see how few rentals are in there and you'll see the reality that it's not the best place for them.
The one option that makes some sense to me is where you find a homeowner that is willing to list the house for sale but also list it for rent at the same time and willing to take whatever comes along. The new option to list rentals in MLS makes some sense for that.