Very old question, but I have a new solution. It is based on screenshots, it wont be usable for all situations, but someone may appreciate it.
Lets say you want to highlight more states in the USA. For example New York and California.
Open 2x Google maps and search both states separately in 2 tabs. (links 1+2)
Then zoom and reposition one of the maps to cover all areas you are interested in.
Now check the URL of the map (link 3). It contains this substring: @39.4563534,-93.9396224,5z
The substring contains info about coordinates and the zoom. Now you have to replace this substring in all the URLs of all your maps. But before you do it, you have to move each map by mouse to any other position, or just change the zoom. If you dont do it, the pasted substring will be ignored.
Now you can take screenshots and paste all the perfectly aligned maps to MS PowerPoint to the same position so they will hide each other. Then use function Format/Crop to crop them. Using this tool you will be able to change dimensions of particular images so that each image will reveal the highlighted area, but together all imgs will create 1 large Google map...
1) New York
3) New York - zoomed out
4) California - with identical zoom and position as NY