From a mechanical point of view it should be flanked = flanked. Driven by the position of the attackers, NOT on some metagamed state of mind.
"I ignore the goblin completely in order to avoid taking a Flanking bonus from his buddy the Ogre."
Ignore a combatant totally? Closed eyes?!? Seriously!?!
Sounds like the character is completely helpless vs that opponent that round... I don't see why the Goblin can't 'Coup de Grace' him (carefully and meticulously opening a femoral artery, say. Colorfully described as 'slicing the bottom off a water balloon'). Or, how about effectively Zero Dex?
"Oh, I'm not ignoring him THAT much!" Fine. You're flanked. Unless I was feeling particularly annoyed, in which case it would be "TOO LATE. Here are some d6s and paper. Go roll up a new character. Next Rules Lawyer please."
Harsh? Maybe, but the whole thing is silly.
Another possibility: How effectively can somebody ignore a goblin sawing away at one's kidney? Sounds like a Concentration Check at the very least. DC = the Combined Attack Rolls of the Flankers seems about right...
Or screw it. Ogre is a 1st level Warrior. Goblin, sadly, is a 15th level Rogue/Assassin and he's been studying his target for the last three rounds. Guess Metagaming didn't pay off...
But all of these are dodges. And punitive ones at that: Flanking should be a condition imposed by the position of the attackers. The whole see/no see state of mind thing raises too many loopholes.
I even think invisible creatures should contribute to a Flank. I think one becomes at least subconsciously aware of the THREAT posed by an invisible creature once they move close enough to attack. The whole gestalt of sound, smell, air pressure, movement etc.*. Not enough to neccesarily target them effectively, but enough to be 'threatened' by them and therefore be flanked. That's just an interpretation though.
A'Mal
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* I've experienced this. Not, obviously, with invisible creatures, but in total darkness situations - both in martial arts classes (a thorough beating was enjoyed by all) and in photographic darkrooms. A person just standing quietly is quite easy to sense. One moving with any kind of rapidity is almost impossible to not sense.
I'm not suggesting that being aware of them makes it easy to defend against them (IBID 'thorough beatings'), just that they would 'threaten' quite effectively...