I understand why the question would be asked. The 49ers and Seahawks love to sign former players from the other team and San Francisco needs help at receiver.
Yet, I don't think Rice would be a high-priority for the 49ers unless he came very cheaply.
Rice is talented, but he can't seem to stay healthy. The 49ers will likely draft a receiver in the early rounds, so there wouldn't be much for him.
But, again, taking a cheap flier on him to see if he can regain his form would be worth considering.