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: Border Crossing Vienna #2: 2H IPA III
Secondary #1: Berry Rhubarb Wine #2: Black Currant Wine #3:Cider
Kegged: Orange Pale Ale IV, Hidden Lake Nut Brown
Bottles: Spruced Winter Warmer, Surly Bender clone, Big 50 Barleywine, Framboise Lambic, Dark Belgian Strong, Kicked by a Moose Scotch Ale, Russian Imperial Stout, Sparkling Elderflower Wine, Barolo Wine, On Deck: Ice Shanty Bock