I use a Black Diamond Bod harness. I find it comfortable and it packs very small. It also doubles as a spare harness for bad weather sailing because it fits on over everything. Many bits of technical climbing gear are very useful for mast work, particularily sit harnesses. I would say that they are at a much more mature point of design than harnesses made by sailing companies. I note that most riggers use climbing gear when working aloft.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217