tried also adding those to top-level, with no luck.

Definitely should be at the top level, this doesn't work?

#kaocha/v1
{:plugins [:preloads]
 :kaocha.plugin.preloads/ns-names [...]}

plugin is really simple

(defplugin kaocha.plugin/preloads
  (pre-load [config]
    (when-let [ns-names (::ns-names config)]
      (apply require ns-names))
    config))

but... I guess you're trying to preload cljs code? that won't work

but you might be able to use the clojurescript compiler preloads option

#kaocha/v1
{:tests [{:id   :unit
          :type :kaocha.type/clojure.test}
         {:id   :unit-cljs
          :type :kaocha.type/cljs
          :cljs/compiler-options {:preloads [...]}}]}