It does but it's extremely cumbersome if you have densely tagged images, as you need to send each image once per tag. Plus I believe there's a limit to the size of a ZIP.

Not sure what you mean here - With Watson Visual Recognition, you can train a set of custom classifiers from your example images.

Then in a single API call, you can upload a batch or single image and get a score for each image for each class in all your classifiers. There are limits to the size of the ZIP files you can upload, that's true.

Maybe you were referring to the _demo_ of custom training, here: https://visual-recognition-demo.mybluemix.net/train ? The demo only classifies using one classifier at a time, but the API can do multiple at once.