|Do you know where your variables are?|
Another way to proceed would be to hack on the browser side. If you're using Firefox, an Autofill extension might help.
If you are using, or are allowed to look at the source code for the AMANDA backup software, it has a complete and working implementation of using the changer's built-in bar code reader to determine which tapes are in which slots. (Overland's web site says that bar code support is built in.)
I checked the Overland web site, but couldn't find any reference to an API.
If you go the packet-sniffing route, be prepared for at least some encryption. The "BrowserID" parameter looks like a session ID or possibly part of an encryption key. Check to see if it changes from time to time, always increases, etc.
Of all these options, I think your best bet is to talk to the changer as a SCSI device if possible. (I'm assuming the device is connected to a server via SCSI.)
Spring: Forces, Coiled Again!