I think it would be fair to request a status from your lender/broker, and if he is a professional, he would also be willing to personally communicate this to the Realtors on both sides of the deal. If you're not getting that level of service, I would suggest you place some effort there before approaching the seller.
If you were in the seller's position, you would want to have reliable information on which to base your grant of any extension. You should work to be able to provide them with that.
Good luck and let me know if I can help.