I'm not sure, but I think you are saying the multiple paths in question here have different bandwidth available. So in that case you don't want to equally distribute the traffic.
ScreenOS has PBR (Policy based routing) that can be used then to distributed traffic based on the source, destination or ports of the traffic similar to security policies. You would need to monitor the traffic and know about how much is there for some of these criteria then create rules to push the traffic out specific upstreams.
This is covered in chapter 6 of the Routing guide
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_Routing.pdf