Instead of deb.devuan.org: debs.devuan.org or deb-ssl. or deb-https.

Then every server needs the same certificate for deb-ssl.devuan.org or you get warnings/error about wrong certs.

Could there be a devuan (web/deb) server with ssl that provides for every package request a "Content moved temporary to https://dev.otherserver.org"?

Every server needs the same content, the same package versions and the same package list. That sounds more like a https proxy. Would it be less work for the devuan server to get just the https request for a package/packagelist, answer with a moved temporarily and the proxy answers with the data or requests the data from the devuan server and answers with the data.