First, I must declare a commercial interest.
My very small company (www.celticmagic.org) is in the process of developing a new product that may help here. My products are generally spin offs from my work from 20+ years as a Remap engineer.
This pending product (GCM100) is a general-purpose box that emulates a mouse / keyboard or joystick and has (optional) relay outputs that act as switches into other products. It can be programmed by anyone but can also be shipped preconfigured in your case a as a mouse controller.
It supports two miniature light force joysticks and twelve switch inputs along with optional breath (sip & puff). Only one joystick and one switch input would be required for your client.
The GCM mouse control software has inbuilt scanning to allow the user to select Single left click/ double click, click hold , scroll etc from the one separate switch although I’m working on a joystick ‘only’ version of software that will do it all from just the joystick in the future.
Q) Can your client operate a standard 3.5mm switch perhaps with his other hand?
The mini joystick is on a highly adjustable arm that can be supported from a range of acrylic bases.
It would be easy to arrange the joystick to be mounted cantilevered from the front of a desk say on the right-hand side and the selector switch on the left perhaps. I’m sure I could supply a base for the joystick to do this if you can provide photos and dimensions.
Production units and the website are not quite ready, but I can offer to give you a prototype for assessment and on permanent free loan if successful. This would be good for me to get a prototype into field service. I only have a non relay version prototype spare to offer for free I’m afraid.
If relays are required the production relay version will be circa £230.
Here is a video of a Remap client using such a device in gaming mode with a very ‘personalised’ switch array arrangement which tends to be a requirement for gaming.
Please drop me an email if this is of interest.