Well, as you say, the Skype API is unsupported but is still in there... so I played around a bit and got something that works (today, with version 7.5, with the moon in its current phase).
The following script is poorly written, but works:
set userList to "skypeuser1, skypeuser2, echo123"
set messageToSend to "Hey, it works!"
tell application "Skype"
set chatIDString to send command "CHAT CREATE " & userList & space script name "skype_messages.scpt"
set chatID to character id 35 & (word 2 of chatIDString) & "/" & (word 3 of chatIDString) & (word 4 of chatIDString)
send command "CHATMESSAGE " & chatID & space & messageToSend script name "skype_messages.scpt"
Basically, what you're doing is creating a new chat session, which returns the following string:
"CHAT #yourusername/$c14a201aac174943 STATUS MULTI_SUBSCRIBED"
The trouble is the CHATMESSAGE command wants only part of that string, from the hash mark (#) to the end of that hex string. AppleScript was giving me headaches about parsing that string as words to get the bits I wanted, so that's why there's the code where I save off the parts I need, and then rebuild them as a chat ID.
And, as far as I can tell, the name of the script in each Skype command can be anything, but it has to be there.
Hope this helps,
P.S. Almost forgot... you can get the Skype API reference guide from the Wayback Machine at http://web.archive.org/web/20130607130426/http://dev.skype.com/desktop-api-reference#COMMANDS