Most champagne yeasts are "killers," that is they produce a protein which cuts off other yeasts. Therefore you will stop your ale yeast strain. Furthermore, champagne yeast on it's own can't break down maltotriose, so it will leave your beer underattenuated...by how much, I can't say, because I've never tried it, but probably by enough to make for a bad beer, since no one does it. You can either use amylase enzyme to free up fermentable sugars for the champagne yeast, which can get the beer below 1.000 depending on how much you add, or you can use brettanomyces to finish the job, since killer strains don't affect it.
You might already know, but definitely keep the sugar under 20% of the fermentables. It's quite common to use 2 lbs on a tripel, but I have zero experience as to how good a sugared stout or bock would taste.
Finally, don't underestimate your ale yeasts. These strains are thoroughbreds created through centuries of human brewing. WLP001 can go to 12% or over. The guidelines, according to Chris White, are just for average fermentations. Extra oxygen, ramping up temperatures, and agitating the yeast can take a beer to very high alcohol levels.
