<$list filter="149 54 9 21 +[nsort[]]">
</$list>
This works because the default handling of a filter is to treat it as a tiddler title. tiddler titles in filters don't have to correspond to existing tiddlers. So the list filter is being passed a list of tiddlers. Then the "+[" sytax means, run the following filter run using the previous tiddlers as input. So now we run nsort against them, getting back a list of tiddlers in numeric sorted order. From there, you can format them however you like.
~Matt