Filtering Custom Taxonomies

I take it this is not possible?

it probably is, but it might be complicated, and I have no experience of using custom taxonomies. However a google gives me this page/post a few times

https://www.ads-software.com/support/topic/custom-taxonomies-conditional-tags?replies=13#post-1110167

You could add a version of that function to your functions.php, and use it in some widget logic. You’d have to change widget logic ‘load point’ to at least after the theme (and hence functions.php) is loaded.