Sorry, wrong analysis of the problem. This change to /usr/lib/sysusers.d/dnsmasq-base.conf works:
u! dnsmasq -:nogroup "dnsmasq" /var/lib/misc I.e. the quotes in the GECOS field are needed for opensysusers. According to the systemd sysusers.d man page quotes are always needed, so this should be a dnsmasq bug.
