That twin-boom canard design looks like it would probably have something going for it in real life.
Re the BD-5S sailplane stability issue, one thing that occurs to me is that, since the engine isn't there, you could fit an extended boom into the tube where the prop/jet used to go, and mount the tail surfaces on it, thus putting them further back for greater leverage and also moving the C0fG back a bit to compensate for the loss of the engine. Of course, structural strength would be an issue, as would the lack of space for control linkages. Perhaps the latter issue might see them going back to a V-tail?