Upon further study:
https://en.m.wikipedia.org/wiki/Radiant_cooling
I think that I could use the buried outside loop to cool the downstairs slab (1440ft2) using the Pex loops designed for heating (7” OC)
Cooling needed about 4 weeks a year.
Climate is dry so this might work, I would need to avoid condensation.
It would be easy to tie into the radiant slab loops with a flat plate heat exchanger & pump the buried loop controlled by a greatroom thermostat and a dew point controller.
I haven’t found any heat pumps under 1/2 ton?