My apologies - for being lazy. The question was easily answered by pulling the boat out of the garage and dry sailing it with the mizzen in the center step. As it turns out, it seems to sheet just fine running the sheet back towards the bow and feeding it through the main sail sheet blocks. There was very little wind, so a stress test under more challenging conditions is needed, but can't see why it wouldn't work. I did notice that, intentional or not, there is a lot more rake in the mast from the center position. I assume that is intentional and designed to prevent lee helm. When I bought this boat the main mast was raked too far back, such that the sprit would not pass by the mizzen mast on a tack - plus, the mast was binding and nearly impossible to turn or remove. Cutting the hole about 1/2" forward solved that issue.
I think this dual system is going to work out nice - if reefing on the water, pulling the main sprit, rolling the sail two or three turns would be a first step; going with the shorter sprit tied off on a cleat below the tack and rolling the mizzen sail a turn or two would be the second step. Given a chance to reef from shore, mizzen in the center step would be the first choice.