I am not too familiar with z-wave. Is it compatible with x10?
Do you know if there exists a command line tool for z-wave? If so you could use the launch function with parameters in VoxCommando.
Of course you would need the hardware to send signals from your computer to the Z-wave system. Do you already have something that does this?
edit: I found this
http://www.controlthink.com/zwavesdk.htmbut it's not clear where to buy or how much it would cost. Clicking "buy now" does not seem to lead anywhere useful. It does not look like z-wave plays very well with others yet... but I have only done a quick web search. Let me know if you find anything else useful, but unless there is already a software solution like a command line tool I would need to have the hardware and some kind of sdk in order to develop something.