Tags and comments can be added in a sidecar XMP file (it should be named the same as the clip file but with an .xmp extension). Comments are added in xmpDM:Tracks, and tags in dc:subject, as per the following file:

<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.3-c011 66.145661, 2012/02/06-14:56:27        ">
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description rdf:about="" xmlns:xmp="http://ns.adobe.com/xap/1.0/">
   <xmp:CreateDate>2012-03-07T12:22:26Z</xmp:CreateDate>
   <xmp:ModifyDate>2012-03-09T17:28:39Z</xmp:ModifyDate>
   <xmp:MetadataDate>2012-05-15T21:02:33+01:00</xmp:MetadataDate>
  </rdf:Description>
  <rdf:Description rdf:about="" xmlns:xmpDM="http://ns.adobe.com/xmp/1.0/DynamicMedia/" xmlns:stDim="http://ns.adobe.com/xap/1.0/sType/Dimensions#">
   <xmpDM:Tracks>
   <rdf:Bag>
    <rdf:li rdf:parseType="Resource">
     <xmpDM:trackType>Cue</xmpDM:trackType>
     <xmpDM:frameRate>f25</xmpDM:frameRate>
     <xmpDM:markers>
      <rdf:Seq>
      <rdf:li rdf:parseType="Resource">
      <xmpDM:startTime>1023</xmpDM:startTime>
      <xmpDM:duration>1440</xmpDM:duration>
      <xmpDM:comment>Are these credits ok?</xmpDM:comment>
      <xmpDM:type>vidixplore_comment</xmpDM:type>
      </rdf:li>
      <rdf:li rdf:parseType="Resource">
      <xmpDM:startTime>1258</xmpDM:startTime>
      <xmpDM:duration>1130</xmpDM:duration>
      <xmpDM:comment>The best scene in the movie!</xmpDM:comment>
      <xmpDM:type>vidixplore_comment</xmpDM:type>
      </rdf:li>
      </rdf:Seq>
     </xmpDM:markers>
    </rdf:li>
   </rdf:Bag>
   </xmpDM:Tracks>
  </rdf:Description>
  <rdf:Description rdf:about="" xmlns:dc="http://purl.org/dc/elements/1.1/">
   <dc:subject>
   <rdf:Bag>
    <rdf:li>keyword 1</rdf:li>
    <rdf:li>keyword 2</rdf:li>
    <rdf:li>keyword 3</rdf:li>
   </rdf:Bag>
   </dc:subject>
  </rdf:Description>
</rdf:RDF>
</x:xmpmeta>