Manual and automatically maintained aliases

In working with path auto I discovered I wanted per node URL logic. Basically on a per node basis I want to:

- Define an alias that does NOT change on a per node basis - fixed URL. No matter what happens this URL does not change.
OR
- Allow a URL to be auto created AND prior ones redirected - auto maintained

Common:
- If a redirected URL ends up coming into existence, it should supersede the redirect
- If a manually defined alias is converted to auto, then the prior alias should be a redirect to the new
- This should probably trigger '/feed' aliasing as well
- Other conditions?

This seems odd but a few sites I am working on have a mix of fixed and auto URLs. The URL must never mismatch the node and the node title can change for various reasons over time (yea, not great but it happens). Given that the sites are increasingly designed for SEO (sometimes to the exclusion of other factors, like usability) the above provides a lot of flexibility.

I don't think this is an easy enhancement so I am proposing it here for comments before I even think about implementing something like it.