Is there a way to make an individual building indestructible? I found the "Indestructable Buildings" plugin but that only applies to the entire server.

You could try to indentify them through their building id (DecayEntity.buildingID) and cancel the damage based on that. I'm not sure how consistent the id is across restarts but if as far as I know, it is consistent.

I'm not a plugin dev just a garden variety admin, I was looking for either an existing plugin or rust admin commands.

In that case then I doubt there are any console commands. I haven't seen any plugins that do this so you might either have to comission someone to create it, wait for someone to create and upload it or you could submit a feature request to Indestructable Buildings.