One way is to copy the opera.ini file to the "Documents" directory (which is visible from the file manager). If you give the copy a .txt extension, you'll be able to open and edit it in Notes.

Type this line in Xterm once you are @ root.

>> cp ./.opera/opera.ini ./MyDocs/.documents/operaini.txt <<

Now go to "Documents" directory and click on the newly created operaini.txt file. It should open in Notes.

Scroll down the text to find the line:

Change the UserAgent ID to what you want...

Save the changes then open Xterm, gain root access, and then type this:
>> cp -f ./MyDocs/.documents/operaini.txt ./.opera/opera.ini <<



I can't tell you if it will work on the site you want but this method does work for changing Opera settings in the ini file.

Opera's Settings File Explained