RevMen:
That's not the worst idea I ever heard.
Geez, tough crowd! 
RevMen:
The problem that comes to mind is dropping the solenoid plunger back down to pick up the head again would cause a secondary impact that might screw up the test results.
You can set the solenoid to drop so that it doesn't drop onto the hammer but rather stop just a little short so the magnetic attraction lifts the hammer. Or simply put a piece of felt on top of the hammer to deaden the click.
The way I see this built is that you'd make a 2kg cylinder from cold roll steel and have it sliding up and down in a piece of vertically-oriented PVC. Sleeve that pipe with a larger diameter PVC -- I'm pretty sure you can find plumbing parts that allow an easy, adjustable length junction between different sizes of PVC. That would allow you to finely adjust the height. Would make for a very small footprint, although you'd probably need to give it some kind of weighty base for stability which would more than likely include the battery.