There is no establishment of religion when the government allows someone from a local established church, synagogue, or mosque to offer an opening prayer for the event.
I highly doubt if there is an established wiccan coven open for services in the community.
Where can you go to verify that someone is actually an ordained wiccan clergy? You can usually find most thaat most churches/synagogues/mosques have some manner of an ordaining process that proves the person is authorized to represent that faith. Do wiccans have a similar process? Do satanic priests? Do druids?
How about the fellow we have had in our area, who paid $200.00 to some con artist in California, so that sucker could proclaim himself a pastor in the Church of Cannabis. Should he be allowed to offer prayer at the meeting? I think not.
So you're suggestion is that only ordained ministers be allowed to pray at public meetings? No lay people at all?