Could a formal spec act as a constitution for a blockchain?

I think you are missing the point. For me, the crucial role of a constitution is to cover areas which cannot be enforced by the code or achieved via economic incentives.

Apart from that, a constitution could serve as a formal declaration of intentions for the code, so that we can easily differentiate between bugs and features, but for me this is only a minor role, as the whitepaper does it quite effectively.

Hive account@dan , do I understand your approach correctly?