Honestly that sounds like a lot of work to end up with a less than ideal solution; that being said I have not tried anything like that, the closet I came was moving cool basement air into the rest of the house via the central duct work (a failure, on warmer days the basement didn't cool fast enough and too much moisture migrated down with the warmer air). Window AC are not built to move air through a duct of any length, even until the furnace fan can take over. Rigid ducting wouldn't be seasonal and would likely need sealing to have any chance of working, flexible ducting would offer a metric ton of resistance.
I looked and it seems an 18,000 btu window unit (at least one example) is ~130 pounds and smaller units, in the 5-7,000 btu range, can be in the neighborhood of 40 pounds.
Here are some things I would consider doing instead:
Get a few 5-7k units, new the inexpensive ones cost ~180, and you can find them cheaper on CL. Or the very efferent DC inverter u shaped ones made by Midea start at 345 (for an 8k, but that does come in close to 70 pounds). Then I'd place them in a few key rooms (living room and bed rooms for example). You could recoup some of the cost by selling the 18k unit.
Alternatively I'd consider a portable floor unit with two hoses. They might weigh more, but they normally have wheels are only a plate with the hoses needs to go in the window. Dual hoses is important, with one hose the air the is exhausted comes from inside your house and outside air finds its way in to replace it.
It a big step in price from the other two, but you could also consider a Mr. Cool mini-split or add on coil (if you are happy with that brand).
