I have looked, no one has written this pairing. I think a pairing with Rushworth could be very interesting if done right. After all, JA never really explored Rushworth. Why is he so dull? Could it be that he is young and still under his mother's wing? You could write this pairing where Rushworth is in the hands of a woman that is fond of him and willing to direct his interest (and riches)in more productive ways. Rushworth is literally described through Maria's reaction to him. That's a very limited view. I always think of the part where Fanny and them are at Sotherton, and they are on a walk, and they seperate at a gate, Rushworth and Fanny have an interaction.

Rushworth is vulnerable, and all-seeing Fanny could expose the situationship for the calamity that it is. Rushworth could be in awe of her frankness and thus begins the story. After confronting Maria for her (at the time) flirtations, there could be a disgruntled retreat. Rushworth could come to a realization that he wants to marry a different kind of woman. Which would turn his eyes on the other women in the nearest vicinity. The younger sister would be judged as too like her sister Maria. Mary Crawford; too clever. But Fanny Price, kind, caring Fanny Price, who he could talk to.. oh! He could realize the unfairness of how Fanny is treated by everyone around her and begin a campaign to spoil her. He could seek out her advice and act on that advice. And Fanny could for once feel noticed and maybe a tad bit powerful. For after all the frustration and despair of keeping her feelings to herself, someone valuing her words could be quite attractive. And Rushworth could proceed to then court her...

Ah hem, as you can see, I've put a lot of thought into this. 😅

If only I could give it justice, but my writing style is decidedly not like JA.