Turntable rotation with Y-axis up add-on Blender is hardwired to work with the Z-axis pointing upward, which is pretty inconvenient if one wants to import the game engine's physic simulation to a third party framework which happens to have the Y-axis pointing upwards.

While the vertices data can be exported to most 3d modeling formats swapping two axis with no problem, the game physics exporter doesn't allow to swap the axis.

One solution is to rotate the view to place the Y-axis up, whis is my approach, but it has the inconvenient to mess the turntable mode rotation, which main axis of rotation is aligned parallel to the Z-axis.

So it would be very handy to have a turntable rotation mode with the main axis of rotation aligned parallel to the Y-axis.



Update:

Most third party game engines, would need two files to be exported, the vertices data (.obj in my case), and the physics simulation data (.bullet)

Objects can be modeled independently of the axis pointing upwards, as long as the exported data have the right axis up, there wouldn't be any problem.

The built-in physics simulation can't be exported with the axes swapped, let's say it is Y up hardwired.

Then there is the third party game engine, which happens to use OpenGL's Y-axis up convention.

So my approach is to leave the third party game engine, and the Blender's physics exporter untouched, then export the vertices data as is, without any axis swapping.

The only problem (sort of) is that it would require to use blender with Y-axis up, which at this point means to be able to navigate "naturally" with Y up.
space_view3d_turntable_y_up.py
Enable turntable rotation with Y axis up.
space_view3d_turntable_z_up.py
Enable turntable rotation with Z axis up.    
space_view3d_turntable_any_up.py
Enable turntable rotation with any axis up, the axis that is the most vertically aligned is the up axis for the turntable rotation until a new mouse gesture, i.e. until either the mouse button or the keyboard key modiffier is released.    


All addons have the keyboard combination by default to 
Ctrl+Left Mouse Drag because there is no need to install them all.

Neither disables the built-in view rotation, so that one can have both the trackball (built-in) and the turntable (addon) at the same time mapping them to diferent keys.