I seem to remember something similar to that also, I know it doesn't take much. I have also read that some commercial brewers use yeast from the Krausen for Carbonation (Because it is the most active). If you do use a separate yeast for carbonation, the question then becomes. What do you do with the remainder of the yeast? If it was Nottingham, you could just seal up the package, but if it was a liquid yeast that would be a different story. Do you make a starter before hand for a lager and pull off 5ml? I guess you could go online and find some sterile vial's and add 5ml and some sterile water to each of them for pitching that way you would not have any waste. Basically yeast washing on a much smaller scale.
As far as a source, I will look through some of my books, but it was a long time ago. So it may be a futile search. I seem to remember that it was in "The New Complete Joy of Home Brewing by Charlie Papazian" but I can not be sure of that.
Cheers
Preston