Is radiant heat in a concrete floor unpleasant in a super-insulated house?
I suspect that if it is cold enough to require any heat, even if the furnace only runs a few times a day, that the slab would stay comfortable.   If your climate is such that you will go extended times without the furnace, you may be correct that it will feel cool.

Also, if you have south-facing windows, you may be able to get some passive solar gain using the slab.   We have an upstairs bathroom with a dark stone floor that gets afternoon sun (from a window that goes down to floor level) in the winter and the floor can be noticeably warm for a while after sunset.