RE: Outta my league here ....
Never having owned or sailed either a land sailer or iceboat I'm may be out of my league. I'm sure I read somewhere that both types sail close hauled on most points of sail. If that's the case a 1/4 scale servo would only have to hold the boom in place and the degree of rotation is not quite so important. It would still require pretty high torque to move the sail but I'm not sure about the slippage. I have had some slippage with a sail as small as 600 sq. inches even with a so called sail servo. I notice too that most iceboat and land sailers give great advantage to the sheets by double or triple blocking for leverage. That might require less power to hold the boom. Sort of like putting the servo in low gear.