The "prototype" is "complete"!
I say "prototype" because I may not get around to building a next version. This has a cardboard structure that I had originally intended to be a non-flight prototype only for dry fitting parts (the flight version to be build out out foam-core), but it seems "good enough", so it may be the flight version.
Visible in the photo are the Arduino Uno, the servo, a mini breadboard with the BPM085 pressure sensor, and to the extreme right a sliver of the parachute (white, stowed). The bent paper clip is a pin that holds the door closed, and the servo pulls it out to allow the parachute door to open. The blue rubber bands push out a plate to force the parachute out. Not visible is a 4 AAA battery pack that supplies the power. The components are lashed onto the cardboard structure with fishing line, which worked surprisingly well. There is a switch, accessible through the open parachute door, that allows turning on and off the power. The USB cable can also be inserted into the Arduino for downloading recorded data through the open hole in the top (the USB cable is just stiff enough to insert without a long nose pliers).
The entire assembly has 175 g mass. This replaces a 20 g "gravity" deploy system, for a net gain of 155 g.
I say "complete" as it is probably ready for a flight test. I have run many tests, carrying it up my one flight of stairs to verify that it detects "launch" (a 1.8 m altitude rise), and "descent" (a -1.8 m change), deploys the parachute as expected, and records data. It has also survived a 3 m drop test (the height of my modest stair well) with no damage.