This is possible via a custom dnsmasq configuration, which would have as far a I know also worked in v5.
Add the following to /etc/dnsmasq.d/servers.conf
server=/domain.com/1.1.1.1
server=/other.com/1.1.1.1
This will forward all subdomains of domain.com
to 1.1.1.1
You need to enable MISC.ETC_DNSMASQ_D
( Web > All Settings > Miscellaneous) for this to work.
This was already pointed out by Bucking_Horn in the original post with the link to GEOIP + DNS multiple resolver - #4 by Bucking_Horn