1st Make the netscreen side policy based with the snoicwall.
2nd. upgrade screenos to 5.4r2 and use policy based routing. Setup another phase 2 auto key ike with the sonice wall ( share phase 1), on the netscreen. Then instead of using static routes to route the remote network into the netscreens use PBR.
3rd option 'upgrade' the sonice wall to a netscreen ;)
BTW juniper has a good tech discussoin fourm called jcommunites off their website.