Edmund Blackadder
Scooby
Of course the only evidence that Buffy COULD have opened the portal would based on the theory (the wide theory) that since Buffy and Dawn had the "same blood" that when Dawn's started flowing, Buffy could throw herself in, her blood would stop flowing (because she died) and the portal was either happy or confused and closed.
In which case, if Buffy's blood is enough to either satisfy or confuse the portal into thinking that the "key's" blood stopped it, then one would have to assume she also could have opened it.
Remember, the "key" was not always a person. If they had made it into the Energizer Bunny, it would have opened when the bunny started and closed when the bunny stopped. (Which may have been never and we'd all have been doomed...thank God they went with Dawn and not the bunny. Dawn was more fun to watch than Anya freaking out all season too!)
Buffy couldn't have satisfied OR fooled the portal if the key had been the bunny. It only worked because Dawn was a human, her sister, and supposedly from her blood. The evidence would be that she DID close the portal...the rest is less evidence than logic, that if she could close it, she should have been able to open it.
But, since there is more than one weird hole in S5's ending...it could be nearly impossible to ever get an answer. Even asking Joss could be iffy. I bet it's hard, even with writers like Mutant Enemy had where everyone is really good, to "get" what's in everyone's head at one time. There may be 5 versions of what THEY think happened.
See, this all works except the question of how did Buffy get any of the Key energy in her, and the moment when Dawn is bleeding and Buffy touches her just really doesn't feel right, its not long enough.
People seem to be confusing 'Summers Blood' with the Key energy. This isn't the case, Dawn was the Key, the energy was all through her body BUT was drained when her blood drained.
Even though it is the same blood type Buffy has, it still isn't the same blood.