Replacing the cable with line is very possible (might need to use Spectra or other high-tech line due to space considerations on hte winch). However, to convert to cockpit control is not as easy! I suppose that you could run the line vertically thru a small tube from the post to the cabin top (unfortunately making it impossible to fold the table) then through a thru-deck block and aft to the cockpit, a lot of bigger boats use that type of system. The later 25s and the 26 had the CB pendant attached further aft (ie: closer to the lower end of the CB) and that made the routing of the pendant to the cockpit easier, basically an almost straight pipe from CB trunk to cockpit, the pipe was angled aft but I don't think there was much of any bends.
The 1975 models 25 and the early 1976 models had the post in the cabin, this was the same system used on the 1971-74 O'DAY 23.
Joe had the best advice, call Rudy at D&R (508) 644-3001