Back in my day the local telephone company used waxed lacing cable for that sort of thing[1]. These days it seems that polypropylene string is popular (search on "conduit pull string").
You basically want something that is slippery and will tend to not get stuck. I have used Dacron fishing line, but that is mostly because I had a bunch of it laying around.
I wanted to reach you regarding your comment here: https://news.ycombinator.com/item?id=44547866 Would you be willing to maybe elaborate on the problems caused - I've planned to adopt Miro Sameks for an application?
DM me via my about me, if interested. Would be very thankful.