A few years ago we were in a similar situation, with some acres unable to be planted.  We took advantage as best we could with the situation and planted most of those acres to alfalfa in August.  Our crop insurance agent said it was OK to do this.  We still received our prevent-plant claim.

It is always best and wisest to check with your crop insurance agent with any questions or activities you may be considering to do.  I'm confident your agent will tell you it is permissible to plant your prevent-plant acres to alfalfa in August.

gordon