Yes you can log in on usairways.com if the US flight is the first segment of your itinerary, - you don't need a confirmation number - look up the flight number for US (do not enter the UA codeshare flight number) , enter name, day of flight, US flight number and your name and bingo you can check in online. I do it all the time.

If your first segment is on UA you are out of luck and will have to check in at the counter. UA won't let you check in online if any segment is a codeshare - so at least US has a workaround.

If I have a US flight connecting to a UA flight - I'll check in online for the US flight just in case I run late (usually a early morning flight) then get my UA boarding passes at my connection when I have an hour to kill.