I'm planning on brewing this soon.
I don't have a large enough kettle for mashing all grain for this (although I may spring for one), and a full volume boil. And I do BIAB anyways as I don't have a mash tun etc. So going for a BIAB partial mash and then extract enough to bring up the ABV.
Will then compare side by side with a real Old Fezziwig. So long as Sam makes it this year of course (they make interesting choices with their varity packs sometimes).
Have you tried omitting a gal or 2 during steepping? Then rinse the bag into the kettle with the remaining water, which will get you to boil vol . The grain will be out. Depends on your kettle size.