Looks like they're randomly dropping the viewer until they get a hit (I've gone over "searching (600)"). I wonder if there's a faster way to do that with their API?
What I would do is a sort of "computer vision". Map has well defined colors for features, so you just need to check pixels with proper shading.
This should be quite easy as street view availability seems like a transparent overlay image, so you even wouldn't need to distinguish it from other map features.
And even if it's composited with the map image server-side, it should be still quite easy to extract it, just look for pixel diff compared to normal map image.