I've done a bit of extra work in the past (playing the part of a news cameraman on 2 of the first 'Silent Witness' episodes.) It was great fun - although you do need a book to read during the scene changes.
The idea of 'angels' appearing in productions as extras has been around for a while and is a fun way of raising funds.
I would have thought that this would be a great opportunity to observe the making of a WWII film. Furthermore if you do have comments during production, regarding technical/historical accuracy you can nip over to the director and tell him personally.