How to use ha elements when using ScopedRegistry

customelement-test.ts

import { LitElement, html } from "lit";
import { customElement, property } from "lit/decorators.js";
import { ScopedRegistryHost } from "@lit-labs/scoped-registry-mixin";

@customElement("customelement-test")
class CustomElementTest extends ScopedRegistryHost(LitElement) {
  @property() hass;

  static elementDefinitions = {
    "ha-card": customElements.get("ha-card"),  // This works because ha-card is ALWAYS loaded before custom cards (for now)
  };

  setConfig(config) {}
  
  firstUpdated() {
    // Elements can only be added to the local customElement registry after
    // createRenderRoot has run(which ScopedRegistryRoot handles).
    // It's definitely run before first render, so firstUpdated can be a good
    // place to start loading elements.
    this.loadEntityPicker();
  }

  async loadEntityPicker() {
    // Get the local customElement registry
    const registry = (this.shadowRoot as any)?.customElements;
    if (!registry) return;

    // Check if the element we want is already defined in the local scope
    if (registry.get("ha-entity-picker")) return;

    // Load in ha-entity-picker
    // This part will differ for every element you want
    const ch = await (window as any).loadCardHelpers();
    const c = await ch.createCardElement({ type: "entities", entities: [] });
    await c.constructor.getConfigElement();

    // Since ha-elements are not using scopedRegistry we can get a reference to
    // the newly loaded element from the global customElement registry...
    const haEntityPicker = window.customElements.get("ha-entity-picker");

    // ... and use that reference to register the same element in the local registry
    registry.define("ha-entity-picker", haEntityPicker);
  }

  render() {
    return html`
      <ha-card>
        Hello! <ha-entity-picker .hass=${this.hass}></ha-entity-picker>
      </ha-card>
    `;
  }
}

@thomasloven this no longer appears to work with 2022.11bx. The call to window.customElements.get("ha-entity-picker"); returns undefined.