Invisafil may be an alternative. It's 100wt poly, and almost disappears. I've found it very well behaved and use it all the time for machine stitched bindings. I think it comes in about 15 colors, but it's so fine you just have to get close, not match.
--Heidi
http://www.hugstitches.com
http://www.flickr.com/photos/32113973@N05/