Update on my experience with this. I got one or two test backups to work from WHM to S3 and thought all was good, but it turns out it wasn't working as the last backup I had on S3 was from january 2015. Luckily I haven't needed to use a backup from this strategy and I've spotted this before I need it.
I have since got it working again - it seems the problem was the timeout* was set too low (30s which is the default) and changing that to 120s fixed it and I now seem to be getting regular backups as expected. Now I just need to figure out how to get them to rotate and delete past a certain date on the S3 side...
* Please note I mean the timeout setting in the 'Additional destination' that you create or edit for the S3 server on the WHM backup configuration page, NOT the two other timeout values that appear near the top of the page with much larger values.
I have since got it working again - it seems the problem was the timeout* was set too low (30s which is the default) and changing that to 120s fixed it and I now seem to be getting regular backups as expected. Now I just need to figure out how to get them to rotate and delete past a certain date on the S3 side...
* Please note I mean the timeout setting in the 'Additional destination' that you create or edit for the S3 server on the WHM backup configuration page, NOT the two other timeout values that appear near the top of the page with much larger values.