I've been playing around with this myself to get vanity nameservers running. It seems that you can point your private NSs to DH's NSs (or their IP addresses) and it will work.
For anyone in the know, does this type of NS chaining have a negative effect on lookup or response times? For example, namecheap will let you set up custom nameservers which I pointed to DH's NSs:
ns1.mydomain.org -> 184.108.40.206
ns2.mydomain.org -> 220.127.116.11
ns3.mydomain.org -> 18.104.22.168
Then, I point the registrar (also namecheap) to those nameservers (ns1.mydomain.org, etc) for mydomain.org
To be honest, I don't know enough about the whole DNS system to understand why this works, but it does. It's strange because I can't ping ns1.mydomain.org, but they show up in a whois listing and the test site I've tried it on is still accessible.
Does anyone know if I'll run into trouble later?