Phlac, I know precisely what you are referring to. I have Mac mini's that drive plasma displays as media servers and had to solve this problem awhile ago. The change in resolution makes the plasma image look particularly bad.
I use a product called, "SwitchRes X" at
http://www.madrau.com/html/SRX/indexSRX.shtml
The only problem is that the function you're wanting doesn't yet work with Intel Mac's. Yes the SwitchRes X app itself is Universal Binary (UB), but it depends on the Application Enhancer (once called APE) which is not out in UB yet. If, as it appears, you are trying to solve this problem on an Intel Mac, then you will need to wait a bit...
And yes, I'm sorry it costs money. And if you can figure out how to do it for free, then let me know.
🙂
