You know, I have an idea similar to this one, but with some diferences.
First, the marriage was planed since the beggining of XX century by Motoko and Keitaro's ancestors to unite the Shinmeiryu and the Urashima style, but because certain 'curse', neither family could have male offprings for many generations... until Keitaro was born.
Second, Motoko is still Motoko. She have heard of the arranged marriage, and because of Hina, she knows about Keitaro, but her attitude toward men is the same. She only agreed to all this because she was expecting a strong man. But, after meeting Keitaro, she would be disapointed. Only after getting to know him better, she would change her point of view uppon him.
Third, Keitaro is still Keitaro. That is, he is totally clueless about the marriage. Only after going to Hinatasou is that he learns about it. But he won't like it, because the promise he made 15 years ago. That, and Motoko's constant beatings for falling over the other girls, or trying ot peep on them while they are on the onsen. Like Motoko, her opinion on her will change over the course of the months.
Fifth. Everithing about them being engaged is keept on secret. They just don't want the rest of the girls to know about it. Especialy Motoko, since she thinks her image would be ruined if other know about her being engaged to a 'man' like Keitaro. Only both of them and Haruka would know.
And last, Motoko is 15 at the begining of the manga. If my memory doesn't deceive me, the legal age to get married in Japan is 18 for men and 16 for women. That means they wouldn't get married until she turns 16.
This is my idea, but I don't have the talent to write it. And even if I could do it, is almost sure I'd write it in Spanish.