The Logitech MX1100 has many programmable buttons, one of which is part of the thumb rest. It is a right-handed mouse, and this button is actuated by depressing it with the outside of the pad of the thumb. This button works well for me as the "enter" button, but is programmable as double click. There are a two more buttons directly above the thumb that are actuated with the ball and/or pad of the thumb; these work less well for me, so I guess you'd need to feel one in person to see if they would suit you.
Regards,
James