It's possible...you would position an image on top of your map and then remove it when someone executes a search. The Gauge + Web Map sample kind of shows this... it displays a loading div while the map is being created and fades out the loading div/gif when the map loads.
Is there a way to use a static image (png) as the initial map, which is of the entire US within a square frame having Canada and Mexico greyed out?
Once the user submits a search it switches to a live map which is zoomed in?
Not quite what you asked for, but another approach could be to add a coloured polygon layer representing the US. If you used the greyscale basemap, the US would stand out clearly.
Then when the user submits a search, remove the polygon layer.
