I think I understand David's question but here is a conundrum. If you mount the parallel guides as they are designed you do need them to be outside of the work. I am speculating, without having a set to look at, that they would occupy the real estate under that portion of the guiderail you need to have overhanging your work already, that is, for the starting and stopping of your saw.
If, OTOH, they were designed to be above the rail they would then be in the way of the saw, unless they were even further away from the work. This would also make it more difficult to register the cut.
It sounds like what David wants are guides that attach to the left side of the rail and that are essentially flush with the bottom of the rail, such that they could lay over the work. Naturally, the stop mechanisms would need to be lower to catch the work and they would need to be designed to allow registering under the rail for work narrower than the rail.
In general, I think what Festool has designed is really without problems because of the fact you already need guiderail overhang at each end of your work. The only advantage to flush guides would be that they would lay cooperatively on really wide work.