That seems like a non-starter: The average person who wants a better smartphone experience is not going to jump through the hoops of registering their "own app" (which would be a lie) and then transcribing IDs/secrets into their phone.
Reddit could offer a premium membership that allows add-free and API access, and require the third-party apps to log in on behalf of the user. The fee might still turn a lot of people off (although the lack of ads would make it so much nicer), but it at least wouldn't be a technical hurdle. That model seems to work for YouTube.
Or they could just continue having their current premium model that doesn't involve the third-party apps they so despise. Reddit premium is a thing already, too bad both the official app and website sucks.