I thought of the same idea myself, but (as someone who worked on Let's Encrypt with Peter) I don't think Let's Encrypt would be willing to make this choice for all of its subscribers, and I think Peter himself would place a higher priority on web sites getting encrypted than on the web sites paying tribute to him (and increasing the network traffic associated with a TLS handshake might provide a slight disincentive for some sites).

It's a super-sweet idea, though!