While it doesn't have a specific seated mode, it has several options available to allow you to paint seated.  In the canvas settings, select to have the canvas be the wall. Move your VR avatar to be positioned so that painting while seated is comfortable. You can choose the 'lock' symbol on the canvas to move it around. You can adjust the size using the canvas settings until it is perfect. For example, I used this to paint sitting cross-legged on the floor. We all have our own definitions of comfort.