The Brembo thumb operated rear brake was famously developed for Mick Dohan when he badly broke his right leg and was unable to use the rear brake with his right foot. Since then it has become the preferred option of many MotoGP riders and is available for purchase.
The thumb master cylinder can be used on its own to replace the foot operated rear brake completely, or it can be used in conjunction with a special foot operated rear brake master cylinder so both brakes can work independently.
Being a MotoGP product, this is a custom option and doesn't come with a mounting bracket of any sort - this must be designed and made by individual race teams for their own bikes.