That's the thing with the 7800: if you want unattended batch operation, you need to use rolls. If you don't like rolls, then I'd argue the 7800 isn't for you. Like John said, consider the alternatives with a big sheet feeder.
If you only print 24" wide every few months, consider sending out the big 24" prints to a lab like West Coast Imaging, Calypso, or Pictopia.
General suggestion is to get a printer that is optimized for your common printing needs (i.e., day-to-day prints), not for the occasional print.
Cheers,
Eric