I do a lot of injection mold design and a fair share of the parts I design have energy directors for ultrasonic welding. My experience is mostly with polypropylene, polystyrene, and polycarbonate. I have not tried to ultrasonically weld a printed part, but the idea hadn’t crossed my mind until reading your post of course. I find the idea intriguing, but anyone that knows energy director design knows it is a sensitive process. I am inclined to think that anything learned from experimenting with ultrasonically welding printed parts would ultimately not apply to a similar injection molded design.
It’s a great question though, and I think the next time I am working on an ultrasonic joint design I will print a part exactly like the molded part and experiment a bit. It would be worth the small effort to see if there are any similarities. The ability to prototype ultrasonic joints would be very valuable and would save a lot of time and money iterating.