You never have been able to trade event Pokemon on the GTS. The attached ribbons block them. Never tried the SPRING Magmar that came without a ribbon temporarily though.
To answer
K1RO, the other Wonder Card pokemon are tradeable. As a matter of fact, when I see people begging for a Zard stone or Chic stone, I trade them the Powersaves hack along with it. Heck, even the badly done, original torchic edit trades over wi-fi. Now the last time that I did this was some time ago, but as long as nothing has changed, the event wonder card edits trade over Wi-Fi like normal (except the "fake" Diancie one.)