The filter in the <<list-links>> macro is used to *select* the list of links to show... however, the filter is not used to *format* the output. The <<list-links>> output format is always a bullet list, and there are no macro parameters that you can add to change that.
However....
The <<list-links>> macro is just a specific use-case of the underlying <$list>...</$list> widget syntax, which gives you much more control over the list generation and output formatting.
In your case, you could start with something like this:
<$list filter="[all[missing]backlinks[]]" variable="missingTiddler">
<$link tiddler=<<missingTiddler>><$text text=<<missingTiddler>>/></$link>:
</$list>
Note: the above example is just to illustrate the use of custom formatting for the <$list> widget output.. but I'm not sure you have the right filter syntax for what you are trying to do...
Try this:
Missing tiddlers and the tiddler(s) that link to them:
<ul>
<$list filter="[all[missing]]" variable="missingTiddler">
<li><$link to=<<missingTiddler>>><$text text=<<missingTiddler>>/></$link>:</li>
<ul>
<$list filter="[<missingTiddler>backlinks[]]" variable="backLink">
<li><$link to=<<backLink>>><$text text=<<backLink>>/></$link></li>
</$list>
</ul>
</$list>
</ul>
Hopefully, the above example will give you a start on using the <$list> widget, which is one of the most powerful bits of functionality in TiddlyWiki.
Let me know how it goes...
enjoy,
-e
Eric Shulman
TiddlyTools: Small Tools for Big Ideas (tm)
InsideTiddlyWiki: The Missing Manuals