Yet operating with an APS-C-body to me is useless when I attach a fullframe lens to it because all I ACHIEVE is a crop by FORSAKING cropping options and carrying useless extraweight with a lens that could be smaller than it is.
I am sure many enthusiast photographers like me suffer from the lack of APS-C-Teles with a long range. But I cannot see any reason for the other two lenses not to become absolute money-makers to whoever builds them first.
First off, until Canon releases professional bodies in the APS-C format, it's very unlikely we'll see an EF-S L lens, since that would be a professional lens that would not work on Canon's professional bodies.
Beyond that, the simple fact is that there is really no significant savings to be had, either for Canon or for the customer, with an EF-S telephoto lens.
Consider your suggestions in terms of the iris diaphragm (physical aperture, i.e. focal length / max f-number) size required:
EF-S 280/5.6 L IS USM - 50mm
EF-S 250/2.8 L IS USM - 89mm
EF-S 500/5.6 L IS USM - 89mm
The elements in front of the diaphragm must be large enough to fill it with light and avoid significant vignetting, i.e. a bit larger than the diaphragm itself. Consider the design of a telephoto lens, e.g. the 300mm f/2.8L IS:
Most of the weight is from the elements in front of the iris diaphragm, and the very costly UD and fluorite elements (the latter can take a year to grow the crystals) are also in front and therefore need to be the size they are. Making the elements behind it which are already relatively small, even smaller so they are sufficient to fill an EF-S image circle instead of an EF image circle, would amount to only a minimal reduction in weight.
Fast L-series telephoto lenses use a lot of glass, and therefore are expensive. Canon will not spend the R&D money on such lenses, which would have to be priced only slightly less than EF versions, but would be an inertial barrier to owners of such lenses for upgrading to FF.
So, my opinion is that while people would like to see EF-S telephoto lenses, for the presumed weight saving and/or cost savings, those savings are not practically feasible, and we'll never see such lenses.