yea sorenson seems to be working fine, i haven't tested it on the actual display but it seems ok so far.
As far as why i need these custom aspect ratio's / resolutions. I work for a company that designs and installs custom displays, video walls, etc etc. We designed a 6'x6'x6' floating rear projection cube with seamless video wrapping the box. The playback software needs the 4200x1050px clip separated into individual 1050x1050 clips (one per side of the cube) So thats why i need this crazy size.
if you are curious we designed the largest seamless rear projection video wall in the US. 120'x11'. It was installed in the IAC building in NYC.
http://www.vimeo.com/280373
http://trollback.com/IAC/
http://iacbuilding.com/interactive/content.html