I really don't see why this should be so rare. There seem to be quite a few machines that should be capable of doing single needle chainstitch out there. http://www.sewingmachinery.com/used/usc-specials.html
Anyway, I had my Belstaff boots fixed up by the shoe guy in University Village (he's now near the key booth in his own building) and he works using an old treadle (foot operated) Singer that appears to have only one thread going in. I can't look in there to see the stitching (he put a shifter pad on the left toe) but I'd guess it's chainstitched. His machine looks a lot like this one from the site above, only with 1 needle instead of 12:
Anyway, he might think you're nuts for asking, but you could bring a scrap of fabric and have him stitch it and see if it is chainstitch. His English isn't so good so just asking probably wouldn't get you anywhere. He's reasonable cheap too, $10 I think for the shifter pad thing including a scrap of leather from his stash. You would have to iron the crease of the jeans yourself though, since he doesn't have that stuff.