Graybeard and ChristianJ - thanks for reminding me of the slot antenna. In '68 they were still under development as dielectric loaded slots by one of my professors at Univ of Mich. I'd had the impression they were basically designed for VHF at the time.
You got me looking. It's apparently a vertical slot on the lower portion of the VS structure. That would be slightly lower efficiency. But, it doesn't much matter. Communications to DAKAR are possible. And at 6.6 MHz even that antenna would be noisier than any (sane) radio connected to it. This reception is not antenna limited. The pattern might be a problem. I'd expect that always to be a problem rather than specific to AF447's situation. (Does anybody know of a good picture of the antenna unmounted from the aircraft? I'm curious precisely how it is constructed. It's not what I grew up calling a slot antenna. But, with the one picture I have of it, there's not a much better short name for it.)
And finally it bears no importance to the AF447 situation because DAKAR did not have the plane in its list of planes to listen for and Brazil never tried using SelCal when calling the plane. The lack of communications was procedural rather than somehow related to an antenna that doesn't work well.
ChristianJ, those gray patches don't look like the diagrams I have managed to find.