Is it possible to define an Android Virtual Device for use to develop Google Glass apps (until the devices become widely available)?
There isn't an out the box emulator.
You can use the playground to preview your cards, this will show you how your card will be laid out depending on what attributes you provide in your timeline post object. On the right. Just switch that view on the right to HTML if you want to provide a custom HTML template for your cards.
As a matter of fact, I just saw an Engadget article (this morning I think) that announced that the Google Glass API is now live
Additional links from the Engadget article:
However, as the developer site lists, there is nothing specific for Android. :-(
(Source: https://developers.google.com/glass/downloads/)
The supported platforms at the moment are: