Harness
You got the right idea Pete, and the 'ladder' is called atrier's (should be two) Bob V same as the 'mast mate' in the pic made from webbing (but much shorter) and used for strict 'aid' climbs for rock or mountaineering.The 'Mast Mate' is a good system but back it up with a 'prussik-knot' round the mast so if all goes wrong, or you pas out from heat, exhaustion, it will hold you. Climbing harness is a safe way to go, keep it as simple as possible, spring-loaded rap device? Getting down is no worry, how FAST you get down is.The one comment I can offer, true, is that I have had two main halyard shackles break while sailing, I don't believe it is umcommon, I do not use them for my halyards any longer.