Finally I found a list of supported MXML metadata tags. It was a struggle to get there. When I started to write this post I wanted to say, that I can not work on this without a list of all tags. But there it is. I am fine.
I hope a first beta will be available soon. But do not blame me for being that slow. 35°C are so rare here in Germany. :-) The last problem is that my beta3-version will expire in twelve days. I am not sure how to work on this without the Flex Builder. And if I can not use it, it makes no sense for me any longer.