Unless their bylaws prevent them from doing so, SAG should have the good grace to either donate or lend their RWR material to the library. But I don't think we can expect graciousness from the Hollywood crowd.
Let's say we are the communist SAG group for this example.
We find we have original George Washington letters that within years could fetch numerous millions.
Do we:
A# Be gracious and donate them all to the Presidential library at no financial gain for the benefit of the nation?
OR
B# Pull a Mrs. Martin Luther King and attempt to gain personal wealth for the family or organization, and to then suck funds from for personal gain, even though you come off looking like a low class creep?
SAG is IMO much closer to answer B.