The IPv6 addresses on the tunnel itself do have names set up by HE. HE controls the reverse zone - the names are static.
The IPv6 addresses on the routed subnet(s) do not have names. You may set up the reverse zones for the routed space via HE's DNS portal.
As far as what names to set the routed reverse zones to point at is defined by you. I do not believe that there is any naming structure that HE accepts here, and without your own domain name(s), you are out of luck.
However, I do note that there are a few DNS hosting services out there that allow dynamic names and such, and they work by users setting up sub-zones and/or host names under their domains. These will not be under DNS.HE.NET's control.
The following is not an endorsement of any service:
Check out "afraid.org" (shared subdomains) and "no-ip.org" (free dynamic DNS for 3 host names) as examples of solutions that may work for you (or possibly not). There may be other similar services....
Alternatively, I note that the registration for a domain name (under some TLDs) may still be less than $10.00/year at some discount registrars. A domain name is preferred if you intend to receive e-mail.