And the idea that champagne yeast will dry out a beer is a myth. It can't attenuate any sugars that brewer's yeast can't also ferment. If there's a gravity drop after repitching with champagne yeast, it's because the initial pitch stalled out above the attenuation limit of the wort.
I think this myth comes from the idea that champagne yeast has a higher listed alcohol tolerance than beer yeast, so if a beer yeast gives up because it has passed its alcohol tolerance, then (theoretically) the champagne yeast can keep going.
I think this idea is flawed because a) most beer strains can go quite a bit higher than their published tolerance if they are taken care of properly - well beyond the alcohol level all but the largest of barleywines and b) unless you pitched a big, healthy, hungry starter, I doubt even champagne yeast are going to be happy when pitched into beer that is already 12-14+% ABV - if that is truly the reason the original yeast gave up the ghost.
If you really thought that you maxed out the alcohol tolerance of your original yeast, then you'd probably be best off with something like eau de vie or distiller's yeast. But even then, I'm not sure if they'd be much help if they're not getting to the party until the beer is already in the double-digits.