Ah, nearly forgot that there was a good article on "poster size rasterization" in Nvidia GPU-Gems. U can find it online here:
http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter21.htmlThey suggest a viewport based approach that generally makes it possible to render at infinite resolution. Ofcourse u have to do the copyback stuff on ur own.
If u have the patience to implement their approach or find another one on ur own it would be great if u could share it here