Miami does need a veteran backup quarterback, so they will indeed sign one. Who that is will be the big question, though; Kaepernick would fill that hole.
It’s worth noting that new head coach Mike McDaniel left San Francisco for Miami, but before he did, he was part of a staff that drafted Trey Lance to succeed Jimmy Garoppolo.
Lance and Kaepernick have similar play styles, so that could potentially work if that is the route McDaniel wanted to go.
I also think having Kaepernick, while it would be a circus, could help Tua Tagovailoa. He would take some of the heat off of him in the media and be a mentor on dealing with so many haters and supporters.
The biggest question for the backup in Miami this season is whether they can win games if Tagovailoa is inevitably out with an injury. I think Kaepernick would be able to do better than Jacoby Brissett, which may not mean much to some, but I think he could win a few games when needed.