I suppose you could create a separate Downloadable but I see no particular advantage to that. Would seem easier to add a new top-level section to update-center.json and thus to UpdateSite.Data. I would suggest something like:
"message": "This plugin allows all users to do anything they like and so it is not safe to install."
"Versions 1.2 and older of this plugin are known to initiate meltdowns in the Acme reactor core. Please update to 1.3 or above right away."
Since I happen to know that you have access to a vendor plugin which provides customized update sites, I would encourage you to prototype delivering comparable metadata from that plugin, or work with someone who could do such a prototype.