Sure, you could train one by computing your own selective search proposals, and then use the ImageNet 200 category detection challenge data.
You could probably actually just use the existing 1000-category ImageNet model to detect all 1000 classes and use some heuristic that if the max probability is low, then the box counts as background. This will be missing bbox regression and won't do as well as a properly trained model, but at least it has the right number of categories.