Thing about a single volunteer is it means the whole veracity of the system depends on one person remembering which beer is which and not getting that wrong. And having to carry the drinks. And answering awkward questions from the barman. Better to have a team, I think.
I had been thinking of a half-pint of each stout each -- that amounts to two pints of test stout per person, which I don't think is unreasonable. Pass-the-pint is the alternative, but might not be as fair. [i:34zdvm6o]And looks weirder than the already-weird alternative[/i:34zdvm6o].
Sneaky forward-tasting will ruin your tastebuds on the day. Do your research now.
(Wet dogs have much more aroma and flavour, btw.)