Can you make a travel region polygon with with Google Maps API?

Google's tools do not provide any way to do this kind of thing built in. While you might be able to do this by routing to a sufficient number of locations and checking the time, another tool that you might be interested in is Graphserver. GraphServer is a multimodal trip planner, which can take data from OpenStreetMap and other data sources. Some of the gallery images show growing shortest-path distance routing, and this is based on a similar metric.

The Google Group would be the appropriate place to discuss the possibilities of using this tool.

Note that this is not a pre-baked tool; it will likely require some investigation and work to get it to solve your problem, but the tool can be used to do it.

Take a look at the Mapnificent API.

Mapnificent provides dynamic public transport travel time maps for many cities in the US and some world wide. You can use the Mapnificent API to augment your Google Maps application with public transport travel time overlays.overlays.

Take a look at London here.

Have you tried the Route360°-API? It can be used to display a layer of travel time polygons on Google maps and it is free. There is a short tutorial on how to use the API. Maybe that helps.