If I open the XMP file in Notepad, and type in:
dc:description
rdf:Alt
<rdf:li xml:lang=“x-default”> Ferry view on the way to Shelter Island; eastern Long Island Sound</rdf:li>
</rdf:Alt>
</dc:description>
the caption shows up in the raw file thumbnail in digiKam. Don’t want to do this for several hundred raw files, though. 