Has anybody used the foundation excavation as the start point for a Geo system.

You certainly can but in this instance I prefer vertical loops as you lose the benefit of solar gain that assissts horizontals in heating dominated climates.
A DX horizontal bed can fit in 500 SF/ton and possibly fit completely under the building it serves.
Insulation is key between slab and loops.
Joe

We do this in FL on tight beachfront lots - vertical bores beneath the slab.

I doubt it would work real well with horizontal, and if the footers are located at a standard depth to get below the frost line but a geo system then affects the frost line and freezes water in soil under footers or slab - could be a problem. Soils that expand and contract right beneath a footer are to be avoided.

I'm in upstate NY the depth below ground of the footers will be about 7-7.5 ft . There will be about 270 lin ft of footing at that depth.

The footers are placed at that depth likely to avoid frost heaves and damage according to design, local habit, or prescriptive code. If you introduce geo lines right below that depth you may freeze soil below those footers, and the result could be structural failure you really want to avoid.

I'm not saying it can't be done, but you need to be sure that the design accounts for ALL eventualities. Upstate NY has a long and brutal winter - extracting your house's required heat from immediately beneath it may not be a good plan.

To put it bluntly, don't do it without expert local design assistance.