What you’d need to do to expose custom filters is to add them as calculations. I realized ash_json_api had no good way to accept calculation inputs, so I’ve added it as an available key when building a filter from input on a field. This is currently on ash main branch. I haven’t documented it, but honestly we need a whole guide on building filters from input. Given a calculation like this:

calculate :name_matches, :boolean, expr(trigram_similarity(name, ^arg(:search)) > 0.7) do
  argument :search, :string, allow_nil? false
end

filter[name_matches][input][search]=foo would do it, which is equivalent to filter[name_matches][input][search]=true&filter[name_matches][eq]=true.