I wonder if you need to insulate that much. Still air is R5 per inch
for
downward heat flow so it would seem a radiant barrier would be enough,
maybe
with a small R-value since it won't be 100% reflective and will get
warm.
I was planning to try this with 1" polyisocyanurate board across the
bottom
of the joists and nothing in the joist bay except the pex.

There is also 'local wisdom' here that heat spreaders can be replaced
by
higher water temperature. It would seem that hot spots would be the
only
downside and your idea of hanging the pex would reduce that as the air
at
the top of the joist bay above the pex would get heated convectively,
whereas
the air beloww the pex would be stagnant and thus R5/inch.

HTH