Can confirmed passengers preview or request flight plans before the flight?
Yes, and you don't even have to be a confirmed passenger (that would be kind of weird). There are probably several sources for this, but FlightAware includes filed route information for upcoming flights. Note that this link won't work very well in a week, since it's datestamped to the future. However, they show the route on the map, and also print out the current planned route:

UAL1
KIAH STYCK1 DOLEY ADM J21 IRW HYS LBF RAP GGW YEA YEG 5600N 12000W BINGA NCA11 COHIL WUXAN 6000N 16000W 5900N 17000W 5700N 18000E ONEIL R580 OPAKE R580 OLCOT R580 OATIS Y802 ARMOR Y810 KETAR Y811 VEPOX Y811 MELON RJAA