Thanks for the advice. After some more fiddling, I was able to get this working. It appears that the off-screen buffer was not copying to texture parts of the buffer that were not occupied by some sort of geometry (and for whatever reason, moving the window remedied this).
The solution that ended up working was to create a model on the off-screen buffer that was larger than the entire buffer, and then quickly dispose/scale down this model. This seems to cause the entire buffer to copy into the main window.
Or something like that. Anyway, it works now. 