You bring up a pet peave of mine on a lot of newer boats. Between wheel steering planting you at the aft end of the cockpit and cabin top mounted travelers I find myself running the length of the cockpit in order to play the mainsheet. It drives me crazy and there are so many more blocks required on a mid-boom set up that you are really working a lot harder to overcome friction as well.
Anyway it is posible to run the mainsheet back to the cockpit by running the sheet outboard to the base of the shrouds and then aft along the deck. This set up is standard on the Beneteau 40.7''s except that the traveler is set up right in front of the wheel. In the racing package the 40.7 has a 2:1 mainsheet and so a winch is required. (I crew on a 40.7 doing mainsail trim.) I don''t know enough about your setup to know if you will need to add a winch back at the cockpit. You should be able to set up the mainsheet as double ended with one end at the wheel and the other adjustable at the cabin top.