Hm. Suspicions confirmed- the objects are flattening themselves to bilboard against the heading of the rendering camera. Popping a cam.transform.rotation tweak into the UpdateCubemap function:

function UpdateCubemap (faceMask : int) {
if (!cam) {
    var go = new GameObject ("CubemapCamera", Camera);
    go.hideFlags = HideFlags.HideAndDontSave;
    go.transform.position = transform.position;
    go.transform.rotation = Quaternion.identity;
    cam = go.camera;
    cam.farClipPlane = farClippingPlane; // don't render very far into cubemap
    cam.enabled = false;

    //cam.cullingMask = ~(1<<4) & m_ReflectLayers.value; // never render water layer
    //cam.cullingMask = ~(1<<8) & m_ReflectLayers.value; // never render self
    cam.cullingMask = m_ReflectLayers.value;
    //cam.depth = 2;
}

if (!rtex) {
    rtex = new RenderTexture (cubemapSize, cubemapSize, 16);
    rtex.isPowerOfTwo = true;
    rtex.isCubemap = true;
    rtex.hideFlags = HideFlags.HideAndDontSave;
    renderer.sharedMaterial.SetTexture ("_Cube", rtex);
}

cam.transform.position = transform.position;
cam.transform.rotation = transform.rotation;
cam.RenderToCubemap (rtex, faceMask);
}

seems to eliminate the visible symptom (yeah, there are some extra commented-out lines in there that I was playing with to solve ~other~ issues).

I'm still curious as to why the engine does this to begin with, and whether it happens on all renders or just cubemap renders...