The low tech answer, if you've got company in a few days, would be to stick a bucket full of water next to the toilet seat and use that for flushing. Of course after flushing you'd have to fill the bucket again, but you've got a 250 gallon tank next door. Maybe stick a tap (faucet that is) in the side of the tank to make it easier filling the bucket.

My dad used a bucket for flushing for months when he moved back to Ireland and they didn't have the running water sorted out yet.

Maybe there's some helpful pointers in here - The Specialist