I think most hybrid classes can pre-stack DI like that. I'm not sure if boomkin loose something if they shift in and out of form, but they could theoretically cast a rejuv or two, then shift back in right before the fight starts. Same with SPriests I'd guess, though I'm not sure if they have a hot outside holy spec. DPS bonus would be minimal but you could threaten to move DI to someone else if they don't pre-load it