Conditional/3d Joint limits

Joint/Axis limit values are accessible from hal but need a queue buster operation that resets the motion planning. So one can change the limit values using an M code for example but limit violations are then only caught while actually running the code and not when loading the gcode because the motion planner cannot 'see' past a queue buster. This might be a solution for the scenario mentioned by pippin88.
Dynamic limits would require some sort of real time calculation of limit values depending on joint position preferably done already when loading the gcode into the preview. There is currently no provision for such functionality.
Collision detection for more complex machines like robot arms might be best achieved by running a simulation using an stl model and checking for interference in the model. ROS uses that if I remember correctly. Linuxcnc uses Opengl in it's vismach simulations but as far as I am aware OpenGL does not support 3D stl interference checking.

I can imagine a HAL component that took, as input, an STL file (for example) and set the feed-override to zero if the tool tip exceeded the defined volume.
But there would not be any warning, the machine would just stop.