Is it not possible to forward two ports for bitcoin? E.g. 8333 and 8334 and announce this ports in the distributed database, like bittorrent does?
No. You could modify the source to use a different port, but "stock" Bitcoin greatly prefers to connect to peers on port 8333, so it wouldn't do any good.
of course, I was thinking about why this is a bad thing, all that I can come up with was to draw parallels to how tor is built.... able to specify bridge nodes, and specific IPs that you are allowed to talk on... and SSL options even chosen to try and minimize the detectability of what is going on.
Though, since bitcoin can use a tor proxy, it gets all those features. How well would it work to use a tor location hidden service as your available IP? If the client just forwards everything through the proxy, letting it do DNS lookups, then it would mean that you just wont resolve for any clients not using a tor proxy, but it should be transparent for anyone doing it.
Of course, if nodes are only advertised by IP (havn't looked at that, might have to soon) that wont work.