Goldentrout,
Under FAR 121 and 135, you are required to touchdown in the first third of the runway (touchdown zone). (14 CFR 91.175(c)(1)). It's tempting to look at the total useable landing distance, or runway length, when considering this requirment. It's tempting to determine if you can stop in the total landing distance, period.
However, if you look on the back of your runway diagram on your Jepp charts, under Additional Runway Information, you'll find the useable runway lengths after the threshold, and after flying the glideslop to touchdown. It is the useable distance after flying the runway to touchdown on the glideslope, that should concern you when flight planning to a given runway. If you believe that this distance is insufficient, then you should not use the runway, regardless of the total length.
Under TERPS, the obstacle clearance plane and distances are calculated for the actual glide slope to the DH. (now DA). Normally this is 200', except for Cat II and Cat III runways. The ILS is flight checked to this altititude. However, below that altitude, the obstacle clearance is only calculated, and the clearance and glide slope is NOT flight checked. Your only assurance of obstacle protection is to maintain the same glideslop angle protected on approach with the glide slope, OR, a visual approach slope indicator. (The VASI/PLASI, etc, will provide obstacle clearance, but may occur at a slightly different angle).
Aside from the other implications, there are legal considerations. To "duck under," you are operating outside the FAR, and you open yourself to liability if anything at all should happen (including enforcement action, for whatever reason). A possible scenario you may not have considered is damage to a trailing aircraft by wake turbulence. If that aircraft was damaged, regardless of what actually happened, you may find yourself on the losing end of a court case because you weren't where you were supposed to be. If that sounds far fetched, it's not. Your reasoning and the specifics may not be all that important.
Ask any carrier pilot about "spotting the deck," and see who thinks it's wise. Landing on such a critically short piece of real estate, the pilot is forced to fly a visual (and electronic) guidance all the way to touchdown. Even a few feet long or short can result in a bolter (missed approach), or a nasty sudden stop at the edge of the deck. Such a landing is a standard approach, in microcosm, with bigger penalties.
Finally, note that FAR 91.3 requires you to become familiar with the runway lengths before departure, as part of your preflight planning information. This information includes the useable runway lengths when landing beyond the glide slope. If this distance is insufficient, the runway is not acceptable for use. Factors such as a contaminated runway, aircraft weight, and density altitude at the destination will affect the ability to get stopped in this distance. You can always select another runway.
If dispatchers don't understand this, educate them.
