You are correct that if you lock the rear freewheel it will turn the front chainring and assist in starting. The problem is that because in coast mode, rear wheel driving the chainring, as soon as you put a load on it, release the clutch, it will just unwind you rear derailleur and your chain will fall off. On your front freewheel system try riding the bike and put some friction on your front chainring with your foot and you will see what I mean.

Pablo is just having a problem because I screwed up and made an incorrect assumption. He caught my error and things should be good to go once he gets the new sprocket.