good idea, although you would need some simplification because you can't test *all* possible combinations:

Each block can have either grass or a melon seed. There are 9x9=81 blocks, so we have 2**81=~10**24 permutations for that field. That is too many to try out.
As a starter, i think we can exclude all rotation or mirror symmetrical combinations. So we only need all combinations for one corner of that field. That would be a 5x5 square, so we have 2**25=~10**7.

If we could try out 100 combinations per second (by using a shit ton of command blocks), we could get this done in ~10**5 seconds which is about 115 days. So still too much.

You could assume that there have to be at least like 5 grass blocks and at least 5 melon seeds in each quadrant but im too lazy to go on with this.

Another Solution would be to create a mathematical model for the plant growth and just simulate the data. Then we could probably get away with a few million calculations since we can try thousands of them per second.