FX sensor cameras accommodate DX lenses, and they do it by automatically cropping out the edges if the image, netting the same result as a DX camera, albeit with a lower pixel count. You don't actually lose the "reach" as some people are describing it, just the pixel count of the D300 at that crop. If you are using a D70, you aren't even really losing much pixel count at all.
With the ISO performance of the FX sensor, you could get longer reach with a converter or a consumer lens and just run a higher ISO to compensate for lens speed.
If I understand what you are asking for, that would require both sensors to be in the camera, which isn't very practical from a cost standpoint. If the rumored 22mp D3X ever see light, the DX mode in that may be 12mp, which may be what your looking for.
It's not an optical illusion. It just looks like one.