I'd use it, so long as it wouldnt have a long ugly url, something simple, similar to placehold.it/300x300, where the 300x300 indicates the size of the placeholder img.
I thought about that. The only reason I didn't do that was that I didn't want too many DNS paths in the way. I'm sure that can be worked around though and I will work on adding that.
Of course, if backgroundhost.com/{slug} is OK then this wouldn't be an issue. I could use a shorter domain name too as long as it makes sense since it is CNAMEd to the CDN anyways.