Set your resource high for SETI@Home and low for the other project(s). E.g. SETI@Home = 10000; Prim = 1.
This means that most of your time will go toward SETI@Home and very little will go toward Prime. Of course, this means on occasion a Prime workunit will slip through, but that's about as perfect as you're gonna get until all projects update their server software.
Once all projects update their server software, you can set a resource share of zero for your backup project and work will only be fetched when none can be retrieved from your main project.