I wasn't sure of the size of the bottom plate and the relative stability once the stands are attached. They'll clearly the top heavy, but if the bottom plate is slightly wider, it might be harder to tip them over.
I did end up ordering up a pair of the 18" matching stands. They arrived over the weekend. I would say they are of average build quality. I was surprised to find the "made in China" label on the box, so I'm assuming these were designed and commissioned before Target was brought under Bryston's wing.
The top plate bolts securely to the underside of the Mini T's, and the same rubber footers as the Mini T's on the bottom plate. Unfortunately the bottom of the tubes are open, making filling them with sand or something else impossible out of the box (you'd need to cap off the end somehow). The base plate is shaped as the Mini T is, with a broad front and slightly narrowing sides in the back.
Stability wise, it does take an intended push to make them tip over. I don't think you'd end up knocking them over with an inadvertent bump or brush. I'll still move them back after my listening is done just to keep them out of the way.