I haven't done it. There is such a variety of microorganisms, and most are not great for making beer. I would be afraid of ruining whatever I pitched it into. chances are over 99% is good yeast, but there are some beer spoilers that don't have to exist in very high concentrations to really mess up a beer.
I would harvest the yeast from the fermentor. The viability will be a function of the ABV level.
The cell density is dependent on the strain and the amount of protein. With most beers and WLP001 you'll probably have about 1 billion cells per milliliter liter.