Further to this: Debian libre already exists and is exactly this. Probably the best way to start. That gives you Libre Qubes v0.0.1 nearly for free.

Then, for hardening: since Kicksecure is also just a hardened flavour of Debian, you could reduce your work to integrating the hardened config and kernel build of Kicksecure into Debian Libre.

Finally: it is worth considering whether you want to be attempting to deblob Xen - if that is necessary. It may make the project unviable as a one-man side-project.

It should be pointed out: deblobbing a kernel does not necessarily harden it. Secure and libre are orthogonal concepts. That being said, I like your idea.