True, "Bigby" has more to do with brand identity than copyright, and that's covered under OGL's brand identity clauses. Gameplay terms (such as Marking) are not brand identity. Besides, since 4e wasn't released under OGL, the OGL doesn't apply.
Copyright, Trademark, and Patents are very, very specific in what they protect. Copyright covers expression (not ideas, not common words). Trademark is Branding, covering registered names and logos. Patents are processes (theoretically, game mechanics can be patented since it is a process). The only real argument in court would be a claim that a supposed infringing knockoff is a Derrivative Work (under that specific clause under copyright law), in which I'm not sure if damages could be collected, but a "cease and desist" could be put into place by order of a civil court. Hasbro, as well as most US companies, are usually fairly tolerant of derrivative works (such as fan art, fan fiction, etc) because it grows the fanbase as well as increasing the viability of a brand.
Still, I'm not one to poke the bear - even though I've thought about co-opting some SW Saga and Bo9S mechanics for something Pathfinder Compatable.