We have successfully upgraded our V2 master server to V3 control server; and installed  Linux Control Daemon on one of our relay server that has connected with the control server. 
So far, I have seen no way to use the control interface to interact with the relay server. Even creating a new account the control server is the only one listed in the drop down box under Hosts.

I saw the import_v226_accounts.sh script in sbin, which states it is not to be run manually, so I tried the same export/import process used on the control server. That gave:
:2199... failed: Connection refused.

Which makes sense since 2199 is the port for cc-web, which is not installed on the Linux Control Daemon. So apparently importing isn't the answer.

I have not been able to find anything relevant in the docs, KB or forum.
So, it looks like my only other option is to uninstall and go with a full install.

When you installed the control daemon package on your secondary server, did you get any errors regarding registration of the server with the web interface?  It sounds like that step failed.

You can manually add the server to your web interface under Management->Hosts.  The "authentication key" is in /usr/local/centovacast/etc/rpcshadow on the secondary server.  Note, however, that if whatever communications problem prevented the server from being registered in the first place still exists, then you won't have any better luck even after adding the server manually.  Make sure ports 2196-2199 are open on both the web interface server and the secondary server in your firewalls.

When you installed the control daemon package on your secondary server, did you get any errors regarding registration of the server with the web interface?  It sounds like that step failed.

Initially, yes - I documented those issues in this post: http://forums.centova.com/index.php?topic=2310.msg6486#msg6486
They were resolved, partly by opening ports 2196-99 on each server.

You can manually add the server to your web interface under Management->Hosts. 

As stated in the above post, after fixing the errors, the secondary showed up in hosts; but still had no control.

We had to have everything back up by monday morning to handle the load, so I installed the full version on each server. All the working streams are using Shoutcast1, so relaying is working as it did with V2 - they are just now independent, with no single-sign-on; I think I read somewhere that that's no longer supported?

When I have the time, I plan on installing Linux Control Daemon on a new server to see if I can get it working.  I'm sure I can get it to show up in 'hosts' with the other servers; but besides the possibility of the host being available in the Host drop down when adding a new account, I have no idea what to look for to verify that it's working.  - in other words, how is the control server supposed to look when it has control of another server? 
With V2 it was obvious with single-sign-on having each controlled server with it's accounts listed below the master server accounts.

They were resolved, partly by opening ports 2196-99 on each server.

That would be why the host wasn't initially registered with the web interface.

As stated in the above post, after fixing the errors, the secondary showed up in hosts; but still had no control.

Exactly what does that mean -- "still had no control"?  Could you describe what happened when you tried to create accounts on that server?

they are just now independent, with no single-sign-on; I think I read somewhere that that's no longer supported?

The new host management system is much more efficient than single sign-on, so yes, we did deprecate the SSO functionality.  It still existed in the code though, and by request from one of our clients we have put it back in the UI in the latest build.  (The page to add remote hosts to the master server is under Utilities, and the option to enable it on the slave servers is on the "Legacy SSO" tab of the admin settings page.)  Only the basic single sign-on login page functionality is supported though; the centralized administration and centralized provisioning features were removed long ago.

When I have the time, I plan on installing Linux Control Daemon on a new server to see if I can get it working. 

While the v3 launch has been a bit bumpy, this area really isn't one of the problematic ones; if everything is done correctly (meaning all ports are open, correct hostnames/IP addresses are used, all directions are followed, etc.) it should "just work".  We have done a substantial number of paid installations since launch, and this part is virtually flawless every time.

I'm sure I can get it to show up in 'hosts' with the other servers; but besides the possibility of the host being available in the Host drop down when adding a new account, I have no idea what to look for to verify that it's working

1. Make sure it shows up in the Host dropdown when adding an account.
2. Select that Host and create an account on it.  If the account is created, then congrats, it's working fine.

in other words, how is the control server supposed to look when it has control of another server? 
With V2 it was obvious with single-sign-on having each controlled server with it's accounts listed below the master server accounts.

In v3, all of your accounts from all servers are shown in the same accounts list.  You can, of course, filter by host using the controls at the top.