Steve:
Would it be possible to try compressed air on the empty conduit? I know that it's a stretch, but the air may either make enough mess or create enough noise to where you can find the other end.
I encountered a similar situation to yours several years ago, but it was about a 300' run of 1-1/2" RMC, about 50 years old. It had a telephone cable in it that we needed to pull out and replace, but it wouldn't budge. No fish tapes, no blowers, no nothing would work so we figured it was a collapsed/rusted-out conduit. We knew that the old cable had to be replaced, hook or crook, so at this point I was more interested in seeing the snake make it through than saving the existing cable.
Due to the property, also similar to what you've described, excavation really wasn't an option. I went for broke and hired Roto-Rooter to come out and give it a try. He was able to run a snake through with a slow spin on it. He obviously knew what he was doing. It turned out to be lots of rust and dirt that the snake managed to loosen enough to not only get the snake completely through, but to dislodge the old cable so we could pull it out. Oh, and by the way, he ran the snake without any kind of cutter head on it; just the raw end of the snake.
I know that in your case, this might not be an option, but I thought I'd throw out another possible solution just in case you run out of ideas.
