a minimal example would be this:

(launcher/eval-str (str '(ns mine
                             (:require
                                       [portal.ui.inspector :as inspector]))
                          '(defn register-viewer! [viewer-spec]
                             (swap! inspector/viewers conj viewer-spec))
                          '(register-viewer! {:predicate string?
                                              :component (fn [val]
                                                           [:portal.ui.styled/div
                                                            {:title "Time"
                                                             :style
                                                             {:display :flex
                                                              :align-items :center}}
                                                            val])
                                              :name :portal.viewer/testing5})
                          '1))

Ah, an explicit eval call. And how to reach from sci into the VS Code APIs I wonder?

you can’t since that ns is not passed to sci

My main interest in customization here is being able to have Portal invoke commands in the host editor (well, VS Code, but I suspect the IntelliJ folks would like this too).

If @U1G869VNV is busy I can try to find some time for a quick prototype, although I've only build custom versions of the standalone ui till now

You can pass the entire JS world to SCI using {:classes {'js goog/global :allow :all}}

Then you should be able to call into any API using (js/vscode.api.foobar)