I tried putting an XMesh object into a Shape Instance Node of PFlow and in the viewport it works fantastically with animation offsets based on particle/event age, but then at render time sadly it doesn’t 
Any chance this could be fixed or is it ingrained into Max?
Thank you for your report!
I want to make sure I understand: do you have a Shape Instance operator, with Animation Offset Keying set to Sync By: Particle Age?
EDIT: Could you please try to select your XMesh Loader, and change the Loading Mode to Nearest Frame Velocity Offset? You can find the Loading Mode control in the Files rollout.
Aha! Success! Excellent 
Great, thank you for letting us know!
(The default loading mode, Frame Velocity Offset, only loads a single frame for the sake of robust motion blur. Nearest Frame Velocity Offset can load multiple frames.)