What happens when the farmland turns back into dirt after having no seeds on it?
Then they wouldn't be able to plant seeds on it, as it would count as a dirt block, but the only reasons farmland should turn into dirt are 1: if someone walks on it, or 2: someone places a block above it.
Perhaps the flag would also allow the use of a hoe on dirt, as long as there is water a few blocks away?
The reason I didn't add this to my suggestion is because it could potentially lead to entire lots being farmed, and some people may not like that. however, the idea of requiring water nearby to hoe dirt/grass might be a good rule for this flag should hoeing be added.