Sim.
A condição agarrada DDI não impede você de atacar o agarrador, então você pode flanqueá-lo se o seu aliado estiver na posição correta.
Grabbed. While a creature is grabbed, it is immobilized. Maintaining this condition on the creature occupies whatever appendage, object, or effect the grabber used to initiate the grab. This condition ends immediately on the creature if the grabber is subjected to an effect that prevents it from taking actions, or if the creature ends up outside the range of the grabbing power or effect.
Immobilized. When a creature is immobilized, it can’t move, unless it teleports or is pulled, pushed, or slid.
Como você percebeu, a única condição que impede o flanqueamento DDI é quando um flanker ou outro é incapaz de atacar (por exemplo: falta de linha de efeito ou impossibilidade de realizar ações).
Flanking. [...] Must Be Able to Attack: You and your ally must be able to attack the enemy, whether you’re armed or unarmed. If there’s no line of effect between your enemy and either you or your ally, you don’t flank. If you’re affected by an effect that prevents you from taking actions, you don’t flank. [...]