Beth and Evans of far way places use this set-up.

http://www.bethandevans.com/pdf/MastClimbing.pdf

I've not used these, but I've had people go up with an ascender as as a "brake" (or safety device.

http://books.google.com/books?id=ptKNLpQ2qrgC&pg=PA220&lpg=PA220&dq=Ascenders+AND+sailing&source=bl&ots=1HsVskQQdu&sig=gwluGEGWB4-KbwIu5UWKPczxr3s&hl=en&ei=IqytTv6QA-PY0QHap7CEDw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CH8Q6AEwBQ#v=onepage&q&f=false