The tab relation definition is like that:

<action id="TAB_RELATIONS" link="/incl/tabs/document_relations.xhtml"
  enabled="true" label="action.view.relations" order="55">
  <category>VIEW_ACTION_LIST</category>
  <filter id="view_relations">
    <rule grant="true">
      <permission>Read</permission>
    </rule>
    <rule grant="false">
      <facet>Folderish</facet>
    </rule>
    <rule grant="false">
      <type>Server</type>
    </rule>
  </filter>
</action>

You have to contribute

• an action with the same id to override and merge the filter you want. I don't remember the merge possibilities about filter, actions, etc… If ones have this information quickly than me without checking the source code…
• or you can just create the same contribution with another id (“TAB_RELATION_FOR_MYDOC” for instance) with the filter you want.