Not sure when Seapointe was added -- but Grand Pacific Palisades has been available as a "direct exchange" to WM owners since 2003'ish. WM owners may book either resort by phoning the WM Exchange desk (vs RCI). The reservations require a 7-night stay and an exchange fee.
I don't believe there are any benefits offered to GPR owners -- unless perhaps there are reciprocal exchanges offered through
GPX (GPR -> WM)??
EDITED to add: Another possible benefit to GPR owners: several WM resorts are offered through
ResorTime as nightly, cash rentals. Access to these rentals might be part of a reciprocal exchange between the two resort families.