How can I fasten 6" XPS tight against concrete slab?
In searching for a suitable nylon screw, I came across threaded nylon rods which are readily available in lengths up to at least 6'.  By cutting such rods I was able to obtain a stem of suitable length.  These were inserted into holes drilled in the slab and fastened using epoxy.  Then, a nylon nut (also readily available) was used to tighten a galvanized washer against the foam and pull it closer to the building.

This was moderately messy (drilling holes in the XPS for the rod and to countersink the nut made a billion little pieces of foam to clean up) and still involved the use of a bit of foam to seal over each nut/washer/rod.

Worse, the XPS was not willing to move all the way to the building in some places.  Where there was less than a 2" gap the approach worked.  For the largest gaps, the XPS began crushing beneath the washer before coming flush to the building.

The remaining gaps led me to choose to spray more to fill these gaps and then, rather than flashing it all with aluminum sheet metal, we used an elastomeric paint over a fabric to extend the house's water barrier over the foam and down to the earth and to provide some mild protection for the foam.  The result doesn't currently look very good but with another coat or possibly two it may improve.

Overall it's not a very satisfying solution but we couldn't come up with anything better.