Being able to upload to a different URL would be a massive advantage to image upload.
I quite often have issues with the backend for the image upload being on a subdomain, but I want the image showing on the main site.
Would be outstanding if we could have an image upload that you can send to a folder on a different URL than the one you are working on.
I very much doubt you would be able to place files on another server without HUGELY compromising the security of that server (you would essentially be allowing anyone to place file there!)
I guess you could put an SC file in a project (or other implementation of an API) on the subdomain and call it using an API step of your Server Connect (probably with some kind of token/passcode to stop any old tom/dick/harry from uploading files to it, malicious or not)