Add the ability to link media when the actual source media is not present
use-case: someone gives me proxy files for media that isn’t already imported in my project (this is a realistic scenario -- someone generates a bunch of proxy files for me and sends them to me over the internet as a way of giving me workable footage without having to send terabytes of data). I should have a way to be able to tell premier where that media is going to be located in the future so that I can actually use the proxies. However, currently I am unable to bring in proxy footage without having the proxied-source footage already present on my machine.

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Byron commented
I believe you can import the proxies then "reconnect fill-res" (right click bin media -> proxy -> reconnect full resolution media) when you have access to them. Premiere will then know what media is proxy media and what is full res.
All you need to remember is that the full res media needs reconnecting before exporting, no way of telling Premiere where the full res media "will be" though