I was thinking maybe the patron could make the weapon "magic" but without any bonuses so it would work if you wanna follow RAW to the letter.
There is no rule that would allow a Patron to do that so you'd need DM buy-in to achieve it. But then you aren't following RAW anyway so then the DM could just allow a non-magic weapon being "transformed" by the Pact of the Blade instead and you'd end up in the same place. As I said before this is an inferior option so I would hope that most, if not all, DM's would allow it.
I still have a hard time to imagine a scenario where a non-magic weapon would be so important to the story that you need to find such a workaround though (possible exception being a double-bladed scimitar I guess).
There is no rule that would allow a Patron to do that so you'd need DM buy-in to achieve it. But then you aren't following RAW anyway so then the DM could just allow a non-magic weapon being "transformed" by the Pact of the Blade instead and you'd end up in the same place. As I said before this is an inferior option so I would hope that most, if not all, DM's would allow it.
I still have a hard time to imagine a scenario where a non-magic weapon would be so important to the story that you need to find such a workaround though (possible exception being a double-bladed scimitar I guess).