The 2 biggest issues i see are:
1) Venting the hot air, out of the crawl space, it makes the unit a lot more efficient if you can do this, if not it will still work, but will have to work a lot harder, and you wont get as much cooling. Typically you like to see ~25-30F delta T across the evaporator.
2) There isn't much of a fan in most of the window units I have seen, so getting it to push up any length of ducting could be an issue. The easy way to overcome this obviously is add a helper fan some where in the ducting.
3) I know I said 2, but the other thing I would do, if you can't have the hot exhaust ducted outside, is to have the intake air, being brought in from the house, that way as it cools off the house it also draws in cooler air. Kinda like using "MAX AC" in a car.
I am not a licensed HVAC contractor, but I am involved with AC for a living. The company I work for specializes in off road AC (excavators, dozers, mining haul trucks ect... I am involved with on a day to day basis prototyping mining ac units that are rated and tested to keep operators cool in 55C (131F) temps. Our copper/copper evap cores will do a 50F delta T.
Swamp coolers are great, easy to run, cheap to maintain (as long as they are made of plastic) The biggest issue is making sure you keep them full of water. We are seeing more and more hiway trucks goto this setup, as well as AG equipment. Offroad stuff, is still too hard on everything and they fail to quick in those areas. On a house where it never moves, That is what I would personally do. Just have a couple big rainwater collector drums, and have that feed a filter and then to the swamp cooler. Throw some fungicide in it and have a nice cool house that is replenished on it's own.