The reason I think you should house rule this is because it is unbalancing and unfair. Let me illustrate.
Fighter 40. He has 10 Epic Fighter Bonus Feats. 7 Epic Character Feats.
Barbarian 20/Fighter 20. He has 11 Fighter Bonus Feats. 7 Epic Character Feats.
Sure its only 1 feat more. But in Epic Levels, 1 feat properly used can be a lot. Non-Epic feats can still be plent effective, and a Barbarian 20 can probably pick up the best ones with a few levels of Fighter. But I wouldn't let him pick Epic feats with a Non-Epic Fighter Bonus Feat slot.
Fighter 40. He has 10 Epic Fighter Bonus Feats. 7 Epic Character Feats.
Barbarian 20/Fighter 20. He has 11 Fighter Bonus Feats. 7 Epic Character Feats.
Sure its only 1 feat more. But in Epic Levels, 1 feat properly used can be a lot. Non-Epic feats can still be plent effective, and a Barbarian 20 can probably pick up the best ones with a few levels of Fighter. But I wouldn't let him pick Epic feats with a Non-Epic Fighter Bonus Feat slot.