Bite-controlled, wireless system with graduated control
Dr. Rüdiger Rupp / Heidelberg University Hospital
A bite-controlled, wireless system allowing for graduated control of the sustain pedal.
Further information seems unavailable, so I suggest contacting the hospital. (NOTE: On 14 Nov 2020 I have done so and will update with any response.)
Head-activated wireless controller
CanAssist / University of Victoria
Head-Activated Piano Pedal:...a two-part technology: a mechanical device that sits on the floor and attaches to a piano pedal, and a headband containing a wireless sensor that measures changes in its own position. The sensor wirelessly communicates its position to the device on the floor, activating it to push down or release the pedal.
Based on the description on the CanAssist website, this seems to have been a custom-built system; however, the website includes a page for requesting technology, so perhaps it can be provided to others. (NOTE: As of 14 Nov 2020, I've requested further information from CanAssist and will update with any response.)
There is a YouTube video of the device being used by its recipient.
Steingraeber & Söhne
For more than 20 years, there have been electromagnetic pedal controls for paraplegic pianists – mostly accident victims – that were invented by the renowned Bayreuth piano manufacturer Steingraeber & Söhne and are custom made. (SOURCE)
The company has sought technical solutions to simplify piano playing for wheelchair users and, above all, to provide them with a serviceable alternative to working the pedals with their feet. (Wikipedia)
Unfortunately, I don't find further information about the system, including on the Steingraeber website. I suggest contacting the company to find out more. (NOTE: As of 14 Nov 2020, I have requested information from the company and will update with any response.)
pianoman.nl / Michiel van Loon
The system is built into their own instrument and adapted to their specific needs and abilities.
The system consists of a solenoid (electromagnet), a control unit for setting the correct forces and sizes depending on the type and brand of the instrument, a 24 volt DC power supply, and a cable set.
The system can be controlled by a variety of different parts of the body, including arm, knee, mouth, and head.
A demonstration video is posted to YouTube, and additional information can be found on the maker's website.
This is the system referred to in the answer by @Michiel and, presumably, in the comments by user32742.
Head-controlled system (no longer available?)
Winfield Clark (dec.)
The Coalition for Disabled Musicians mentions a head-activated device created by Winfield Clark. However, the website is no longer active, I did not find information about the device on the Wayback Machine version, and the inventor is deceased. I mention it here for completeness and in case an adventurous reader wants to try tracking down a family member.