Is it Feasible to Insulate without Removing Sheetrock/Panel/Plaster?

Yes. Here's a great photostream from Flickr that shows such an operation in action.

I've done something similar as a service to clients who are having me put in large-ish crown or cabinetry. Either of these makes for an opportunity to insulate without having to do any follow-up mudwork or painting. I simply make a hole and pour in the little styrofoam pellets.

The link above shows a more sophisticated approach by an insulation crew.