filtering queries using aql_query_vars

This is probably a dumb question, but: are there any more examples of how to use aql_query_vars besides the one in Extending AQL?

I struggle to understand the syntax needed to, for example, dynamically add a taxonomy to $query_args[‘taxquery’].

Obviously having dynamic placeholders would be ideal for this, and a lot less messy, but unfortunately I can’t wait for these to appear (unless it’s planned for very soon).