If your HOA has a Grounds Committee or Landscaping Committee or something similar (some group of owners who can approve, let's say, color of shutters or front door, or design of a mailbox), you might approach that committee and ask for permission to build your fence. Now, technically, your rules don't allow for a 6' fence, but committees have been known to look past the rules in making their decisions. It's not a great way to go about it--your attempt to modify the actual rules was the right way--but recognizing that others have already done so, with the apparent acquiescence of the HOA and the appropriate committee--you might be able to receive approval for what you're seeking by using that technique.
Is your HOA voluntary? There is no security in breeching the regulation that others are ignoring. We recommend standing your ground, remaining loyal to this regulation and posturing for a rule change that makes the HOA guidelines enforceable.
If this is not possible, push to have the regulation stricken from the by laws....
"This above all, to thine own self be true"
The "Eckler Team"