Yes, I understand what you want 
As I explained, it is not likely to happen, since unfortunately from our side it IS a major change to the existing system (which thousands of people have already automated using the existing patterns). So, let’s come up with a different plan that can work for you!
I still don’t understand why you need the link at deploy time. The best practice is to use relative URL’s, so /asset.png rather than https://deployid--sitename.netlify.com/asset.png, which would mean that the site works at any hostname. Could that work for you?
If not, things get hairier but still possible: you could create your own API client against our API (see [Support Guide] Understanding and using Netlify's API for details) to manage things using a workflow like this:
- you’d do a build using placeholders for the URL,
- you’d post to our post to our “create deploy” endpoint, which returns the deploy ID,
- …and be able to modify your files to use it…
- before uploading them.
That’s not something we can give a lot of tech support on, so I’d encourage you to keep looking for a way NOT to need that URL which I look at as antipattern - it’s only really useful/intended for sharing or testing after deploying and shouldn’t be referenced internally within the deploy, in my experience.