ArmA 3 v1.16, 2014-04-29 still not working...
Hopefully x additemToX seems to work as a workaround.
Getting bored of all that workarounds and broken stuff guys .

In fact a run a script witch contains these instructions through execVM into the unit field of the unit.

Also have a look here:
http://forums.bistudio.com/showthread.php?167860-Added-Uniform-Disappears-in-Multiplayer&p=2557447&viewfull=1#post2557447

We tested this with 2-3 guys on a dedicated server, with the following simple instructions into the init field of the unit:

removeUniform unitName;
sleep 1;
unitName addUniform classname;

Without sleep 1, it fails systematically for all players.
With sleep 1, it works sometimes and fails sometimes randomly for a given player joining the server.

This looks like a problem of data synchronisation through the network.
Hope it helps.

Still exists in 1.08

I succeeded finally on 1.10 to join the server with -connect=myAddress -port=myPort -password=myPassword.

Be sure to allow arma3server.exe into the firewall of the server for in and out connections.

Still exists into version 1.02.110654

Yes it is a simple "admin" password.