What does *G have to do with seat selection on partner airlines?
Your *G benefits are listed here:
http://www.staralliance.com/en/benef...silver-status/
No where in that list is guarantee of selected seats or priority seat selection. I don't know any airline that guarantees your pre-selected seat will be the one you actually get a boarding pass for. Many reasons justify seat changes of pax. Voyager members also would have no additional pull in this situation. Just hope for the best at IAD and be polite without acting entitled.