How much work would it be, maybe through an extension and maybe not, to arrange for individual tabs to suspend and resume after being idle for a period of time? On Linux and OSX, at least, this might be done via SIGSTOP and SIGCONT, or perhaps using a more sophisticated scheme. Although it'd come at the cost of Chrome's responsiveness, I expect that it'd be a net win.
For now, I use a script to kill the helper processes and reload the ones I'm interested at the time. Effective, but a blunter tool than I'd like all the same.