FarmStripFlyer, The BMW engine with clutch set up you describe is similar to one in a friend's Skyranger microlight. Seeing the propeller stationary after the engine has started certainly takes some getting used to!
I think the clutch is there to stop the gearbox seeing the torque reversals from the boxer engine at low rpm, so it is probably not a good idea for the engine to be in the transition RPMs between clutch disengaged and fully engaged for extended periods, as clutch slip and gear chatter may occur.
Some experimenting on the ground to determine clutch engagement and disengagement RPMs would be well worthwhile, bearing in mind that the rpm to overcome prop inertia with the clutch engaging will be higher than the disengaging rpm.