On #wikimedia-tech just now Ryan Lane told me, "if you need to make software changes, you'll need to make them in your project," and suggests: https://wikitech.wikimedia.org/wiki/Help:Single_Node_MediaWiki
I may well need to change code in the Universal Language Selector extension to get it to do what I want. So I think Beta is actually not an option and I will need to, as Ryan says above, make them in my own project. Right?
I'll go read that help page Ryan pointed me to, now.
Roger