This does feel like an "extension" use-case, as I doubt many users will want to provide manual regexes, and I'm not sure it's the job of the web browser to select and/or maintain appropriate replacements (what if one of these interfaces turns out to be doing something untoward in future, or sells out?)
Noting your comments on extensions "selling out" and stealing cookies or mining data though, is that the root issue here that leads to this request arising? Perhaps this is where better clarity around what an extension can do is needed? Orion controls what domains an extension can run on just now (with options to be prompted by default).
I don't know if the extension API would make it viable to restrict egress and web requests from extensions, but that seems the next logical step to preserve privacy and avoid data exfiltration from extensions. Even if it was a whitelist that users could review and edit in settings, that might offer some assurance that extensions can't just exfiltrate things to servers of their own choosing (or do telemetry!)
Anyone know if this kind of egress management/filtering is viable in the webextensions API?