Not a good idea!
While it initially sounds like a good idea there are some things you have to consider. the boom and the sheeting system are sized for end boom sheeting. moving the traveler to the cabin head moves the sheeting on the boom forward approx 3 feet and you end up with mid boom sheeting, i think this would put too great a load on the existing boom. i think it is feasible if you changed the boom to stronger cross section. also keep in mind that your sheeting loads will double so you would have to change the sheeting system to have twice the purchase that you now have. you would need to go to at least a 12 to 1 purchase. i have a dual system on mine, 4to1/16to1. these systems are outlined in the Harken catalog. If your interested, i have posted some pictures on this site on "O'Day 30 pics" my hardware setups on QuePasa?s/v Que Pasa?