Problem is that your client side error is nothing to do with the code in question. It's bad data. Get that fixed and try again.
Best thing to do is to try a coop or dogfight mission. Host a game and start it without anyone joining (Both will run without a second player unlike TvT). Once you can get the mission to run on both PCs then you can begin testing in-game joining.
A. "(the) client side error is nothing to do with the code in question"
1. What is "that" which needs to get fixed?
2. What data is "bad", and what does the "good" look like in comparison
3. What method is used to tell the two apart?
In other words, what tells you that (a) The data is bad, and (b) What data to compare it to (there isn't a whole lot of documentation on multiplayer in general, let alone this experimental feature) and (c)
B. Hosted game works, and plays, with one player, or two. Done. That's why we're testing it. It's the stock "The Gauntlet" mission, modified to use our unique ships, and *only* for those ships.
C. Did you see the MANTIS report, with the server-side info?
I have the errorbox code from the client, and the server-side debug spew listed in that MANTIS report.
Could you be a bit more specific as to what you mean? You asked me to go ahead and test the features I wanted to use that were experimental or not-quite-implemented - so that's what we're doing
Bobbau:Yeah, I figured

But, seriously, kara had talked to me about this in another thread, and on IRC.
he said:
I'm happy to answer any questions you have as long as you'll report any multiplayer bugs you find.
So, that's what we're doing

Plus, he said "some testing is in order", earlier in the thread