i dont think so there is a plain simple documented process to integerate breifcase using SOAP
but on the other hand its all SOAP between zimbra client and server and all the communication can be seen using the debug mode of client
try:
Web Client URL Tricks - Zimbra :: Wiki
also try firebug in firefox browser and look at the GET and POST on Briefcase actions clicks in client.
for autentication and other commands how zimbra works there is small documentation in shape of
soap-admin.txt
soap.txt
Raj