I would think your best bet would be to contact a few sign shops. I know there are some solvent printable flag/fabric materials. I was supposed to receive some sample material to play with, but I guess I need to touch base on that as it's been like 4 weeks!
Sign companies will generally have the bigger printers necessary to do this, as 54" wide printers are quite normal as the most common sized sign is a 4'x8', these are the same printers they generally make vehicle wraps with as well. These printers use solvent based inks which generally have a fairly healthy life outdoors (2 years being the norm without a laminate, which the flags probably won't have).
DTG printers will suffer due to the size you need, as they're made for garments and generally aren't wide enough or built to print long enough to do what you're asking. There are some wide format dye sublimation printers that may also work, I would suggest trying a sign shop first though. Dye sub inks are not meant for prolonged UV exposure and will probably break down around 6 months outdoors, varying on condition.