I must say that I do not like this proposal. The current situation does result in under-maintained packages requiring churn, but that's true for many aspects of them, not least their policy version. It's a good indicator of which packages need some attention.
That's not what I dislike about the suggestion, though. I think it makes the cognitive load of the control file larger. You have to know there are special rules that exist for some URLs, but not all. It ties the function of the control file closely to Debian, and if other control users like Ubuntu implement something similar, there are even more special cases for a maintainer to have to understand. A simple URL like we have now is quite self-describing, and if you find a control file in the wild you don't need any additional expert knowledge to use it. If/as rules change over time you will have skew from control files and the current state of affairs, unless you try to version it and then we truly do have something too overengineered IMHO. -- Jonathan Dowland