My BIL is a 1/4" shy of 6'6" and has been riding a Trek Marlin XXL / 23". It is the first bike that fit him properly.

Trek has several other 23" MTBs and might have one close to 1k like you're looking for. The Marlin is pretty basic and runs about $600. I'm thinking the X-Cal or one of the other HT 29ers. I don't think they make any 23" frames for 27.5 wheels, but I could be wrong.