If it were that easy, it would already be done. With s-on it will not let you downgrade, only reflash or upgrade.
I have a theory on how to do it, but not positive it would work, and not willing to try. Especially since im already s-off. It is a method previously use on the htc incredible to downgrade in order to get s-off. It involves flashing an older misc partition image, tricking the ruu into thinking its being reflashed or upgraded instead of downgraded.
Here is a thread on how to do it for the incredible http://forum.xda-developers.com/show....php?t=1600904
It would be basicly the same procedure, but you would need someone still on an older version of android to pull a copy of their misc image. And use rum runner instead of unrevoked.
If any devs see this please chime in on wether or not you think this would work.