I need to manage CPU-heavy multitaskable jobs in an interactive application. Just as background, my specific application is an engineering design interface. As a user tweaks
There's plenty of distributed resource managers out there. The software that meets nearly all of your requirements is Sun Grid Engine. SGE is used on some of the worlds largest supercomputers and is in active development.
There's also similar solutions in Torque, Platform LSF, and Condor.
It sounds like you may want to roll your own but there's plenty of functionality in all of the above.