It seems to me that giving an archer the option of trading a full attack or a manyshot for a single attack with power attack is fair enough. At say level 6, the archer can choose between 1 shot at -6 to hit for an extra 12 points of damage, 1 shot at -4 for double normal damage (using manyshot) or 3 shots, 2 at -2 to hit and 1 at -7 for normal damage (using rapid shot). All three look to me like balanced options with different benefits and disadvantages. Provided you don't allow the archer to power attack on every shot in a full attack, or to combine your power attack with manyshot, then I don't see it as unbalanced. I would suggest making the called shot a full round action, rather than a standard action, if you are worried about it being too strong. The only problems I can see come from combining this new option with spells, such as Storm of Arrows or Hunter's Mercy, but making it a full attack action should limit those options enough. Personally, if you restricted this option to within 30' only, I'd consider it a bit underpowered compared with manyshot, and would be unlikely to take the feat.
Cheers,
quetzyl