I visited the Steinbuhler company's booth last year at the national MTNA conference and spoke at length with them about these. Not only are there 7/8, but other sizes as well! I played on them, and I could reach a 15th. \:\)

The way it works is - you take the action out of your piano and send it to them. They make another action (with smaller keys) and send both actions back to you. The two actions can then be changed at will. To my knowledge, this is only possible on grand pianos.

Southern Methodist University has been doing a lot of investigation into these keyboards. You might be interested to know that Carol Leone (from SMU) is going to be giving a recital at Texas Tech on March 25th and will be playing on a 7/8 keyboard that travels with her.

The faculty at TTU have expressed interest in obtaining a pair of 7/8 size action stacks (one for the piano in the performance hall and one to practice on). They're quite expensive (at least $5000 per keyboard) so the price is a bit of a barrier.
"If we continually try to force a child to do what he is afraid to do, he will become more timid, and will use his brains and energy, not to explore the unknown, but to find ways to avoid the pressures we put on him." (John Holt)