As per the comment in Privoxy’s ticket #2530592, I’ve set up a Privoxy rule to interpret dot-less domains as search terms. This emulates the search function built into Ephy, which breaks with Privoxy’s DNS error messages. In /etc/privoxy/user.action
I now have:
# Redirect dotless hostnames as search parameters to Scroogle {+redirect{s@^http://([^\\.]*)/$@https://ssl.scroogle.org/cgi-bin/nbbw.cgi?Gw=$1@}}