Another possibility is to use an "Etrier" which is a sort of webbing
ladder that you could hang from your halyard. You would make it from
1" tubular webbing with steps tied every 2'. You might even find a
cable ladder from a climbing supply place. However, climbing either
one is harder than it sounds. The admonition about a safety rope
strongly applies here.
For that matter, why not have holes for steps pre-drilled in your mast
but do not put the steps on. Instead, they'd have captive studs that
would slip into a slot with an opening at the top of the slot into
which the stud would fit and then would be pulled about 1/2" down into
the slot. You would then need very few steps as you would climb and
pull up the ones below you as you went up. I dont really think this
would work well.