This is a great idea and it mirrors one that i have along the same line. The use of mobile applications might not be as effective as in many people in areas you are targeting use feature phones. Perhaps you can consider one part of your idea on savings and loans to use a USSD platform which can be accessed by any phone. We have such a solution which we deploy to Micro finance institutions. This brings me to my next proposal.
If possible, you can get the farmers to have one common block and unite together according to regions. These farmers can then start a table banking model with regular weekly or monthly money contributions for savings. Through this structure, they can also get loan each other. Again, from this groupings, they can be able to get better markets for their rice and also explore options for value addition for their rice.
My concept revolves around coffee in Kenya. The farmer takes the coffee to a cooperative society, then we get all the coffee from that cooperative for marketing. We can then advance cash to the farmer based on what he or she has brought in.
If you would like to engage me further in my line of thinking, do let me know and perhaps we can work together.