The existing “tally boxes” in the right margin of Legend do already indicate Mirrors in a way similar to the examples above.
And as already mentioned, I think “bullet”-based approaches are not compatible with Legend’s multiple options for bullets.
There are two reasons I find the “tally boxes” inadequate:
1) the right margin is easy to overlook especially when using narrower Panes with items that wrap the line
2) the tally boxes are not explicitly used for Mirrors; they also appear on Linked Items.
The tally box only appears on-hover for linked items, and linked items do have a unique visual style. So strictly speaking, it is always possible to identify a Mirror visually. But it’s a rather subtle set of distinctions.
Legend’s flexibility means it sees a lot of very different use cases. I can easily imagine that some users would actually want mirrors to blend in and be nearly indistinguishable from standard items. I can also imagine other users would want them to stand out very obviously. I personally would like them to be different in a clear but not “loud” way.
For these reasons I believe the best solution would be to include Mirrors in the user-configurable THEME settings. In this case the current behavior would be a sensible “default” configuration, but each user could define a visual style for mirrors that makes sense for them.
If this were available I would probably apply a CSS “shadow” effect to my mirrors – I really like the idea of a reflection effect but I’m not sure I could make that work since it’s a lot more complex!