Fun times! I have a client with a Sabathil (4, 8, 8, 16), and have replaced many strings over the years. To say it's a pain in the ass would be to understate how frustrating it can be!
That said, a hemostat is your friend when attaching the loop to the hitch pin, and good lighting is also very important, as there can be so many shadows, and trying to trace exactly where you need to run the string can be very difficult.
I liken it to the challenge of replacing a treble string on a piano, which runs underneath the overstrung bass. For that, I use long copper tubing, put the 180 bend in the wire where it will attach to the hitch, and put the two loose ends through the copper tubing. Put the loop over the hitch pin, and clamp the hemostat on the hitch pin and pull the copper tubing off of the strings.
But I digress. Thankfully, harpsichord wire is nowhere near as unruly as piano wire! Best wishes and have fun!
Eric Carlson, RPT
Artistic piano service & sales
632 Hickory Hollow Road
Waterford, WI 53185
262-930-8995