Route-by-route load factors are proprietary information which airlines guard zealously— why inform your competitors about where you're making your money and where there's slack in the system? Indeed, in 2004, now-defunct JetsGo sued WestJet accusing the latter of stealing domestic load factor data, and of using that information to "gain an unfair competitive advantage over Jetsgo and prey upon Jetsgo's business and operations by targeting both profitable and vulnerable routes, flight times and fares."
The best we can do is make guesses on flight-by-flight load factors based on things like available fares. After all, a flight that has plenty of T fares at 21 days out probably isn't that full. Some airlines, like United, offer an "Expert Mode" interface where you can see available booking classes. You could alternatively subscribe to a service like ExpertFlyer, which has the same information for multiple airlines.
I would caution that seat maps are notoriously unreliable guides, as not everyone selects a seat prior to the day of departure. The seat map may be wide open, but that's because it doesn't include the tour group of 24 scheduled to be on that flight.
I would add that steady capacity reductions over the last 5-10 years, enabled by airline mergers and forced by high fuel prices, mean that most flights on network carriers will be very full. As recently as 2005, United would a block middle seat as an unpublished perk for Premier Executives, but I can't tell you the last flight I've been on where the middle seat has been empty, even in Economy Plus.