Not really clutched I guess, more bearing-latch.

Think about the crank connecting to a rotating bearing connected to the chain rings. The crank also connects directly to the chain rings, but through a springed locking mechanism on alignment. If it is unaligned, it doesn't lock; rotate it and it will latch into place when aligned.

So, overall, when unaligned, the crank arms and chainrings spin independantly. Upon alignment, the crank latches to the chainrings, so pedaling will turn them. You put a brake lever on the left handlebar (as the clutch is on a motorcycle), and when the rider grabs it he unlocks the crank so it freespins.

Thoughts?