Roo,
The people who work with wire rope at the electric company in re-supporting the cables have a great idea which seems to comprise one section of the pre-kinked wire rope strand from a regular piece of cable. Taking one of the six pre-kinked strands, they wrap it around the two wires to make a sort of "field-splice" connection - really just a wrap. They hold for a short (undetermined time) period and may provide the kind of temporary, non-slipping wrap you need. The space between the "kinks" would be the size of your two strands or a little less, and you would presumably have to experiment a little to see how many wraps of the pre-kinked strand you need. Big advantage is low-cost and easy application with no kinking of the attached wire or base wire. Big disadvantage as I see it is not knowing how much load it will take before slipping - maybe use some friction tape over the wire to start with? Also, if I remember correctly, wire fence rope is single strand - that could present a problem for the pre-kinked wire solution.
This could be the wire-only solution you may be seeking. You are absolutely correct that the U-bolts would be hard on the wire, either the joined-on or the joined-to piece - not pretty! The double-base clips are rather chunky.....
SR