I just had a look at some gnutella implementations and tried to find out how instant messaging would work in a p2p network.

Turned out most development teams use a rather classic approach. For example frostwire have their own IRC server and phex are using only direct peer to peer chat.

I'm curious: is it possible to utilize gnutella (or parts of it) to build a p2p chat / instant messaging network?

Maybe I should divide the question into three more manageable parts:

1) How would a servant create a list of other servants who are currently online?

2) How would a servant send a message to another servant utilizing the p2p network?
(2.1: direct peer to peer connectons do seem difficult due to firewall / nat issues - don't they?)

3) How could 'broadcast groups' of servants be managed (like channels in IRC or peer groups in jxta)?

Most of your requirements had been realized in DC++, you can see ApexDC++ and Verlihub.

DC++ use a central server to manage the users, and detailed rights for each group.

But the server(DC++ hub) can not manage many users.

Good luck!

Ares Galaxy also has a built-in chat feature, allowing users to host a chatroom.