All right, folks. Didn't take too much digging, and I patched together some info from a few sources, including this forum, and found where in the code you update it.
Quickly:
* in \plugins\content\jw_allvideos\includes\sources.php find the YouTube block.
* in the URL string(s) add this:
So this:
https://www.youtube.com/v/{SOURCE}&hl=en&fs=1
Becomes this:
https://www.youtube.com/v/{SOURCE}&hl=en&fs=1&hd=1
* You can add other YouTube operators there. I added No Related Videos.
So my URL became this:
https://www.youtube.com/v/{SOURCE}&hl=en&rel=0&fs=1&hd=1
The URL is in two places, both the <object ...> and the <para name= ...>
If anyone has anything to add that'd be great.