EDIT: looks like this no longer works. Not shocking given how much iOS has changed in the last 7 years.
I got this working by using a PNG instead of a JPG.
Weird thing is, the JPG worked in iOS Safari locally (via a POW server on local wifi) but when pushed to staging the poster image didn’t load. Both the local and staging code were referencing the same file on S3.
Changed the file format to PNG and it loads fine.