Found it, sort of. Benjamin Jalon had the answer, it just took some heavy searching to find it! From his post here: https://answers.nuxeo.com/general/q/56a44b7100bd425ba26f6394eb790518/How-to-design-a-Tab-in-Admin-Center-in-Management-Tab I found that I could make this entry in Studio -> Settings & Versioning -> Registries -> Action Categories:

{
    action_categories : {
    USER_CENTER : {
      "label" : "Custom Home Tab",
      "type" : "tab",
    },
 }

Then, also in Studio, navigate to Project -> Listings & Views -> Tabs and create a new tab. The Definition “Category” dropdown now contains “Custom Home Tab” as an option. This tab will appear in the home/user center left menu.

============update===============

The above code does add a link on the Home tab, but the link is somewhat broken (have to click the link, then click “Home” to see the content view in the home user center). Adding the following xml extension solves that problem. Note that “MyTab” is the name of the tab created in Studio. The xhtml file in action link is generated by studio, you just need to edit the path and replace “MyTab” with your tab ID.

<extension target="org.nuxeo.ecm.platform.actions.ActionService" point="actions">
  <action id="MyTab" link="/studio_tabs/studio_tab_MyTab.xhtml" label="MyTab" order="50"
      type="home_rest_document_link">
      <category>USER_CENTER</category>
</extension>