OK, I don't know of a way to scrollbar the whole screen, but you can bind Ctrl+dpad or something to scroll around in it. You can also use the pager down in the left corner, above the button-bar. (You may have to push windows down (click button 3 on decorations) to see the pager.)

I used the rdesktop-cli that comes with pyrdesktop, using -g 1280x1024. Then used FVWM to scrollbar that window. Since you only need the large resolution on one screen, it's easy enough to just make a shell script to run it, although that functionality might be nice in pyrdesktop at some point. (Particularly, it would be more widely helpful for specifying smaller sizes, so part of the screen doesn't get chopped in windowed mode... Not that your use doesn't count.)