This is achievable, there are 2 methods:
The easy method uses Qubes OS firewall but does not play nicely with domains with multiple IPs associated to it, or if they change often. Add a disposable qube providing network with sys-whonix as a netvm, and use this one as the netvm for the qube you want to filter outgoing connection. This “sys-firewall” buffer will be able to hold firewall rules as sys-whonix can’t do that. Just add the firewall rule you want in your qube settings.
The other method only supports HTTP but works flawlessly with domains behind load balancers or multiple IPs. This would be to use an URL filtering HTTPS proxy and put it behind sys-whonix, the qube that will use a single domain don’t even need to have a netvm.