It looks like you wont be able to do it unless I missed the spec in the P50 that says roll paper.
The Epson R 220 did accommodate roll paper and banner mode, so Qimage could make a print of enormous length.
I read three reviews of the P50, and no mention of roll paper mode at all.

Fred

I just got off the phone with a friend with an R 220. It has roll paper mode and the spools for mounting. According to him, and comparing to my R 1800 roll paper setup, they are so similar as to be considered the same.
I don't know about User Defined in the P50. I don't have that driver or manual, but as long as there is no roll paper option with banner mode, you will not beat the built in sizes.
You could still have User Defined when you have odd size paper that you cut or just want to define; not larger than allowable.

Best I can do.
Fred

This was on the UK web site. The US site gives much more data. There the P50 is called Artisan 50 and will do up to 44 ins (1100mm) in length, is that enough for you?
http://www.epson.com/cmc_upload/0/000/151/125/C30667_ARTISAN_50_PSPEC.pdf

Terry.

There seems to be some conflict in the info available, see:
http://www.techradar.com/reviews/pc-mac/peripherals/printers-and-scanners/printers/epson-stylus-photo-p50-631972/reviewAs I said, d/l the driver & install and then you'll see for yourself  
Perhaps the Artisan 50 and the P50 are not the same items 
Terry.