You could see if osso-xterm supports the -e flag for exec. Example ...

/usr/bin/osso-xterm -e 'sudo gainroot && other-command-here'

*edit* gainroot worked for me, although I couldn't get any commands which output to actually output anything, although not getting any errors. Also notice there is a problem opening multiple terminal windows without using the menu in the terminal. It add's an extra shortcut bar to the bottom of the main one for each extra terminal you open.

*update* It appears that -e is working as expected, it executes the sudo gainroot as user and then executes ls as user not outputting to the root user. You might want to try a shell script, or perhaps see if using just sudo will work to run whatever command you need to run.