Prefix Delegation is now considered "best practice" for ISPs actually implementing IPv6. Currently, he.net allocates a prefix when you press a button on the website, and that prefix is statically configured by the user. You still want to manually request allocation, since HE permanently allocates them, and you don't want to accidentally grab a bunch.
But the tunnel interface (with *::1 and *::2) should respond to Prefix delegation requests, and respond with (one of) the routed prefix(es) assigned to that tunnel. Correctly interacting with PD should then be one of the IPv6 certification tests.
What's the better test: us logging that your side correctly configured to be a DHCP client and got a range, or that you have the know-how to SET UP a DHCP server and provide PD? I don't think verifying someone configured their host to use DHCP is a decent test of networking/hosting skills.
Both tests are important. The DHCP client side is a lower level test, but is less trivial than you think, e.g. NetworkManager has very inflexible support. You can certainly verify that an address within a delegated prefix can be pinged by he.net - that is also important. The lower level test is important ammunition for users pestering their ISP to support IPv6 already as you have requested.
I can't think of a way for you to verify that a client has correctly implemented the server side of PD. But if I do, I'll post it here.