ElP,
They certainly could, but if they bought under their name then they would have to declare when their total holdings of PRB shares exceed the 10% level (they are now deemed insider). Subsequent acquisition (or disposal) of addtional shares would have to be reported.
One possibility is to ask some of their friendly people to get the shares with an agreement that the friendlies would vote their shares with AEM. I don't know how this would be done, but they and their brokers would know how to do it, lol!
Are you psotulating a creeping take over? Not sure if AEG war chest would still have enough cash for this TO. However, they could do this to have more voting power and to become an insider for better leverage in a bidding war. They may bid up and up, but would eventually accept the other guy's offer (e.g. Gold Corp?) to cry all the way to the bank with tonnes of cash (to pay for Osisko aquisition).
Just my speculation.
goldhunter