I've been building a captain's bed for my son out of some old
reclaimed pine. *Due to the limited amount of wood I was not able to
avoid having some knots at the edge of a face frame. *One of them is
big and really shabby, with quite large voids. *I'd like to fill it,
and I'm thinking epoxy would be the best choice. ....

Another suggestion: Cut away around the knot and patch the wood with a
scrap of the same stock. Try to match the grain as best you can. I
think that would look better than a lump of epoxy.