Yeah, that’s going to be a problem regardless, though I don’t see a lot of people tying into that specific spot (although it’d be cool). One alternative, is to force the outlet to be below the current glyphs, so those who do choose to, know they are going to be extending that user menu.
That also means anyone without any said plugins then wouldn’t get that extra hit.
Something like
<li class='glyphs'>
{{d-link path=bookmarksPath title="user.bookmarks" icon="bookmark"}}
{{#if siteSettings.enable_private_messages}}
{{d-link path=messagesPath title="user.private_messages" icon="envelope"}}
{{/if}}
{{#if showEnableAnon}}
{{d-link action="toggleAnon" title="switch_to_anon" icon="user-secret"}}
{{/if}}
{{d-link path=preferencesPath title="user.preferences" icon="gear"}}
<div class="glyph-outlet">{{plugin-outlet "user-menu-glyphs"}}</div>
</li>
Which producces
And this way, it is only instances that install plugins that add said glyph that would get this behavior, but at least it can easily support 3 of them.