I have a few warnings.
- There's strict whitelist regarding domains that can be used as a source for URL based uploading. For all projects but commons, the whitelist is *.flickr.com and *.staticflickr.com only. Commons has its own whitelist and more websites are included. I think that any request for removing this whitelist (=whitelist all domains) will be vetoed by system administrators, because it can be abused easily.
- By default, URL uploading is API-only. I guess you want this to be added to Special:Upload as well.
- I'm not sure if we can enable upload by url feature on additional wikis. I'd like to see opinions of others. Will ping other people working on site requestsv in our conpherence room
That's a good question. I remember some discussions (althrough I don't know on which task such discussions were) about technical restrictions regarding communication between upload.wikimedia.org and server caring about upload_by_url feature (I can remember somebody said in that discussion it is technically restricted).
Because upload.wikimedia.org is whitelisted on test.wikipedia.org, I've tried to perform a file upload via URL on upload.wikimedia.org and it completed with success (testwiki administrators can verify here). It will probably work in the same way on any other wiki, but - I'm not sure if we can (should) process.
From non-technical side, why would you ever upload images that are just resized? MediaWiki can resize them for you and even if they should be uploaded in separate size separately, they should be uploaded on Commons, because they are free and everything free should be on Commons. Local upload is for non-free uploads only and as such tool it is enabled only on wikis with an EDP (Exemption Doctrine Policy).
That's just repeating what the feature does. Question is what kind of images do you want to upload in this way? What domains do you need? What do you plan with this particular domain you want to be whitelisted?
Currently, the upload by URL whitelist doesn't allow to transfer files from one site to another.
I concur it would be useful, but ops would prefer for that to make the storage backend (the component where files are stored) more intelligent to avoid a local copy only to move from one project to another one.