IMO, such attack may occur when you may have more than one assailant; one grabbing you from behind, while another guy in front gets ready for a shomen tsuki (face punch).
Well, wouldn't it make more sense for the attacker to grab with Ushiro ryohijidori?
Anyway, I think you guys sorted the issue for me. From what I've read, nage needs to adapt to the grab. Just a shame we don't practise something like "Bearhugs" where I train, as it would be helpful in dealing with the real thing, as opposed to winging it when it actually occurs.