Post-M6 - restore X,Y ?

Be careful with the set/clear model, your idea is M6 sets extended limits to be able to reach the tool change? That means in normal conditions your limits are not including the tool change area, this could be a problem if you want to jog there to verify anything, could even prevent you from loading gcode without errors if preview sees the tool change moves, also think about emergency stopping during an M6, you will not get to run the clear command. Just pointing those things out, I would consider having open limits and only restrict them when running and not doing a tool change, that should get you what you are looking for, without any drawbacks.