maya 2012 now supports offscreen playblast on windows that allow high resolution viewport capture without the onscreen limit (previous use of a virtual monitor driver on windows has its own limitation and require viewport move and resize) but there are some problems with the flag cf and defaultRenderGlobals.imageFormat with cmds.playblast to capture viewport as image which used to work in previous versions (* please let me know if it is actually possible). The current workround is to save the screenshot as maya iff and convert it to the desired output file format. see script

playblast2012.zip

image above as jpg at 4000×2250
note: viewport 2.0 behaves differently, need to send it to render in viewport2.0 render for higher resolution, drawing override matrix needs to be adjusted if there is any custom gl drawing