I am wrong on the Internet! Official Word of Fred is that, yes, a "free tag" can be used to Invoke for Effect." My sincere apologies for being so stubbornly wrong on the threads in which this came up. Mea culpa.
Per Fred: "A tag is an invoke (tag just means free invoke); an invoke can be done as an invoke for effect; an invoke triggers a compel, which is run between the GM and the target."