Hi @Glyn
I think the filter ‘acf/fields/post_object/query’ will work just fine, however the taxonomy will not be passed in as a parameter. This means you need to find another way to find the taxonomy.
I would use a function such as get_current_screen
(http://codex.wordpress.org/Function_Reference/get_current_screen)
This function will return lots of information about the current edit page.
Does this help?
Thanks
E