walking_carpet wrote: as a copyright holder, lucas can control what the LoC can screen or make copies for distribution?

I wouldn't consider it control. I would guess out of Courtesy the LoC asks the copyright owner before screening, and if the copyright owner balks or does not give timely permission they do not show the work.

walking_carpet wrote: but he cannot order LoC prints to be destroyed?

The LoC is in no way ever going to destroy a work on purpose. But as what may have happened with the Star Wars given to the NFR, Lucasfilm could have asked for the print back, the LoC returns the print, and Lucasfilm never returns it.

From the link danny_boy just posted: (article written in 1993)

Archives, however, are restricted in certain other uses of the copyright-protected films they physically preserve. (They may make films available for on-site study but without permission of the copyright holder or the transfer of rights, archives generally cannot publicly exhibit copyrighted films or distribute them for sale. For further discussion of these access questions, involving copyright, fair use, and "public domain" films, see Section 8.)

and reading further into this doc, this part talks about how the LoC does re-release Public Domain material from their archives:

The first six videotapes in the "Library of Congress Video Collection," due for public release in December, will make available six early silent features and 29 silent shorts from the Library of Congress preservation program.