If a character no longer meets the requirements for a
prestige class, he or she loses the benefit of any class features
or other special abilities granted by the class.
from the text you sited:
Meeting Class Requirements: It’s possible for a character
to take levels in a prestige class and later be in a position
where the character no longer qualifies to be a member of the
class. An alignment change, levels lost because of character
death, or the loss of a magic item that granted an important
ability are examples of events that can make a character ineligible
to advance farther in a prestige class.
If a character no longer meets the requirements for a
prestige class, he or she loses the benefit of any class features
or other special abilities granted by the class. The character
retains Hit Dice gained from advancing in the class as well as
any improvements to base attack bonus and base save bonuses
that the class provided.
Some feats have prerequisites. Your character must have the
indicated ability score, class feature, feat, skill, base attack bonus,
or other quality designated in order to select or use that feat.
A character can’t use a feat if he or she has lost a prerequisite.
For example, if your character’s Strength drops below 13
because a ray of enfeeblement spell, he or she can’t use the Power
Attack feat until the prerequisite is once again met.
NOWHERE does it say that:
"Meeting the requirements" (a feat for a PrC) = being able to use that feat.
...and not simply having selected/"having"/knowing that feat.
The text explicitly says that you must fulfill requirements in order to SELECT or USE a feat, as it explicitly says that if you lose requirements you can't USE the feat.
NOWHERE does it say that if you lose a prerequisite, you lose the feat altogether.
You can yell and shout and moan, but NOWHERE is it written down that "meeting class requirements" means what you claim it means.
Moreover, the examples stated are the following:
1)Alignment change
2)levels lost because of character death
3)the loss of a magic item that granted an important ability.
#s 1 & 2 are pretty clear, so let's stick to #3 so as to respond to this:
A ring that gives power attack is effectively identical to gaining the feat power attack. If you lose access to the ring, or to the feat you may no longer use power attack.
Saying you HAVE power attack you just don't have access to it is the same as saying you HAVE a ring of power attack it's just been temporarily broken into pieces. In either case, item or intrinsic ability you NO LONGER HAVE ACCESS TO POWER ATTACK. You may not use it as a feat, whether to qualify for a prestige class or in combat.
When a magic item grants an ability, and this ability copies a class ability or feat, the magic item magically grants you ALL the knowledge required so as to allow you to do what it grants you. A ring of power attack grants you the knowledge of an experienced swordsman, thus permitting you to wield the sword in the appropriate way so as to receive Power Attack's benefits. You lose the ring, you lose the knowledge. No longer do you KNOW/HAVE Power Attack.
OTOH, if you have already selected Power Attack as a feat, it means that you have the knowledge and experience in you, that you have mastered the art, which will grant you Power Attack's benefits. Even if you lose some strength, the knowledge sticks with you, it doesn't evaporate as with the loss of the ring (You have already SELECTED/MASTERED that feat. You HAVE that feat).
Saying that you no longer have access to power attack AT ALL... yet can still use power attack (which you can't do) as a qualifier is bizarre in the extreme.
Don't twist the words of others, so as to create an effect. NOONE said that "you can still
use power attack as a qualifier"
What we say is that by not being able to use a feat temporarily, does not mean you lose the feat altogether, as explained above.
What is bizarre in the extreme, is saying that if your muscles get weakened, you lose a swordsman's knowledge.
So, again, not only what you claim is not supported by RAW... It is not even supported by RAI.