Thanks @ScruffR!
Your notes on the Photon’s regulator are much appreciated and have clarified suspicions that this might be related to the way the servo draws power – given the higher polling rates at which digital servos update their positions, all that switching noise on VIN might also have contributed to oscillating the regulator to death as you so vividly described. 
Filtering caps – definitely adding them in next time. I feel tempted to intentionally try this experiment again with filtering caps, while over-driving the endpositions…