Yes, I did read the 8-page State of Washington Department of Fish & Wildlife "Wild Turkey Spring Season 2008" pamphlet, cover-to-cover. It doesn't mention anything about the actual LOAD that must be used in a shotgun. It just must be 10 gauge or under.
The HUNTER SAFETY argument (about a single projectile going long range where camoflaged hunters may be hiding) is reason enough for me to use a "conventional" shotgun pellet load instead of a Sabot Deer Slug. I don't want to get anybody else hurt.
Nevertheless, as far as "spoiling the meat," I would think it would be preferable to punch a hole through the turkey rather than have to pick through small lead pellets in the breast meat.
Anyway, it apparently IS LEGAL (but perhaps unwise or "unsporting") to use a Sabot Deer Slug for wild turkeys in Washington State. Other states allow rifles, so the "hunter safety" risk must not be a primary concern there.