I know this is a bit of an old topic, but figured it was worth mentioning if no one had seen them. Flexcut makes a large line of reciprocating chisels. They seem like with modification to the shaft they'd be great for use with an AirGraver. I don't own either, so I can't say for sure, just thought I'd bring it to everyones attention.
http://www.flexcut.com/
I've been lurking in the forum as a guest for the past couple months, just noticed this thread when it resurfaced. I'm just getting started with engraving, even though right now. I have found Steve's forum and tutorials invaluable.
The work-along post was great, hope you continue them. I'm pretty stuck for the time being with practicing solo, so having some decent references is really great. I'm sorry, but no book, or basic picture online gives a beginner must idea of depth of cuts/etc. Seeing so many in-progress and tips on one practice plate helps fill those gaps.
Thanks -MJ