Hi crumply,
maybe you could use a trace from camera position to the mousepointer.
This is the code I use in my point&click adventure:
pos2.x = mouse_pos.x;
pos2.y = mouse_pos.y;
pos2.z = 10000;
vec_for_screen (pos2, camera);
trace_mode = IGNORE_PASSABLE + IGNORE_PASSENTS + IGNORE_FLAG2;
c_trace (camera.x, pos2, trace_mode);
Now you can use 'target' to place your new entity.
Regards,
Pegamode