I would think that the top tubes on quality mixte frames are strong enough as they work together to combine their stregnth at the head tube junction as seat stays do on "mens frame" seat clusters. The triangulation of the long stays helps too with the connection to the seat tube as they pass it on their way to the head tube, so You're really looking at unbraced leagnths that don't get too long. I suspect that the tubes are of the same gauge that they might use for seatstays too and we know how strong those usually are, even in frames with "pencil" seat stays that are thinner through it's legnth than regular tapering seat stays on most bikes.
At one time, I actually wa thinking of putting a small 4 cycle motor on a mixte bicycle frame to make myself a small custom motorbike because the mixte frame seems to be very similar to some motorcycle perimeter type frames.

Chombi
84 Peugoet PSV