Theoretically champagne yeast should be less attenuative in a maltose environment since champagne yeast has evolved to ferment in a fructose environment.
I have thrown champagne yeast at stuck fermentations on two occasions and both times it did nothing at all. one time I pitched two packets of dry champagne into 5 gallons of RIS stuck at 1.036 and it touched it not a jot.
I thi9nk y9our experience is pretty typical, Jonathan. It's just another of those homebrew myths that refuses to die.
You could be right - I have only used it to bottle condition a well aged monster (Lord Fatbottom from NB), where the original yeast was just plain spent after 3 months aging in the secondary. It finished off the BW nicely, but maybe it just carbed the brew and that made the difference in the perception of the cloying-ness drop....so, I withdraw the champagne yeast suggestion for all but carbonation purposes with these big boys.