My contribution is more a hint than a regular contribution:
Open ackphysx.c and have a look at its content. You will discover many useful things, such as a nice way for 2D physics, for example:

pXent_setbodyflag(my,NX_BF_FROZEN_POS_Y,1);

Now you will have a nice 2D-physics-engine. You can lock the pan and roll angles so objects remain 2D (that is they always face the camera), even if they are 3D:

pXent_setbodyflag(my,NX_BF_FROZEN_POS_Y | NX_BF_FROZEN_PAN | NX_BF_FROZEN_ROLL,1);