Not the parent commenter, but I imagine mounting it so it rotates around the nozzle’s axis would work. Fiber optics would help if the laser unit is too big to have swinging around on the print head.

You might also need to be able to tilt the laser up/down to vary the distance from the nozzle.

It should be pretty easy from there to have the computer keep it pointed at wherever the nozzle is going next.

I was thinking 3 lasers 120 degrees apart, turned on as needed, rather than a mechanical solution. 5W laser diodes would probably be enough power.