What I've now done twice is a sort of external version of Andrew's shutter. I didn't bother with the cable release as the pivot for mine is set to provide enough friction to hold the shutter wherever it's positioned -- sort of always on "B."

Attached is the most recent incarnation - a separate shutter sub-assembly.

(More than you wanted to see of this WPPD 2011 project here. Awaiting some graded RC paper for further testing.)