We are trying to use amazon s3 for our epi image/blob assets, and have written a rudimentary blob provider to handle this.
we can upload files to s3 and they are showing correctly in the cms UI.

however, the reason we are doing this is to be able to resize images on the fly from the front end using query params or something of the sort, having those being served directly from the S3 bucket/image service.

When the image data is returned from the content api, the image path is the globalassets path (www.site.com/globalassets/path_to_image.jpg), whereas we would like to use either the blob uri or something of our own construction to be returned in the json. 
The images are generally set on ContentReference properties on the page/block, and thus seem to come out via the ContentReferencePropertyModel. although possibly would need to handle images which are placed in tinyMCE areas too.
The Url property for this is appears to be generated via the urlresolverservice, but I am wondering if there is a way to override this just for images/blobs, which doesnt involve loading the content to check its type in a custom ContentReferenceConverter/PropertyConverter.
Unless there is a way to control this from the BlobProvider itself, but that just seems to deal with generating the uri and loading the bytes of the blob.

 using contentdeliveryapi 3.3.0 and cms 12.6