OK, with my28' S2 with 48' mast, the max righting torque I calculate is
about 4500 ft-lbs. For comparison, the torque produced by the 3300 lb
keel is about 9900 ft-lbs.

Now, for using expanding foam to silence internal halyard slap:

With mast down drill holes at 3' intervals, make the holes about 1/4"
to avoid stress cracking around smaller holes. Inject enough expanding
foam. While it sets over a few hours, work the halyards back and
forth.

With mast UP. go up[ bosuns chair, drill 1/4" holes. Put a small
balloon through the holes and inflate. Push the nozzle of the
expanding foam in beside the balloon and above it. Inject foam. The
balloon is there to keep the foam from just dropping down to the
bottom.

There are various versions of expanding foam, some exerts a lot of
force/area and can push things out of alignment (not an issue here) and
a less strong version intended for insulating around windows that would
also work in this app.