Similar to your 5 gallon and decanting, but can't you just brew 5 gallons of lower gravity lager?. Send that over to a carboy for lagering and use the yeast cake for your dopplebock? I'd hate to see you decant off 5 gallons of scrapped beer, when you could get drinkable product with a little aging.
Primary #1: Empty #2: Empty
Secondary #1: Belgian Golden Strong #2: Dark Belgian Strong #3: Barolo Wine #4: Framboise Lambic
Kegged: RedHook ESB clone, Fat Tire Amber
Bottles: Surly Furious clone, Kicked by a Moose Scotch Ale, Apfelwein, Russian Imperial Stout, Trappist Dubbel, Carmelite Tripel, Pinot Noir, Rhubarb Wine
On Deck: Kölsch IV, Altbier II