I hadn't tried to tie a prusik around the mast, good idea!
One other way to tie yourself on to the dynamic line (the one going to a winch) is to tie a figure 8 with a three foot tail, put the tail thru the attachment point on your harness/bosuns chair, then take the tail and tie a "follow-on" figure 8 where the tail traces the lines of the figure 8 and you wind up with a figure 8 with a bight. This way you don't need a carabiner.
