Right, but you don't get Visualize (or any other other screen savers) when running in 360 extended mode, do you? At least, I don't recall ... ;-)
Honestly ... burn-in won't occur unless you're really sitting on a still image for a very long time. I run MCE on my Pioneer Elite 1080p plasma, and find the screen saver (really a very dark screen saver) that kicks in after 6 minutes does a good job. I do wish that you could configure a shorter time to activate, though ...
One other thing about plasmas ... they're hugely inefficient in terms of energy. I used to turn the television off when I wanted to play music for hours (in the end I'd much rather have a constantly-moving screen saver over the static dimmer one they give you, and since I can't when running with the 360, I would turn the TV off), but have now found it easier in terms of energy to simply switch inputs on the TV to one that's not being used. :-)
Final suggestion here ... you can always run a picture slideshow during music playback.
Steve