Tell me why the Forsaken wouldn't be able to obtain a undead dragon?
The article on undead dragon's on wowiki pretty much confirms they could..
"Undead dragons are the remains of fallen dragons that have risen and/or been withered through
necromancy or other means and originated from the different dragonflights. They can appear usually as reanimated bones. Some are controlled by the Scourge under the Lich King,
while others are raised or controlled by other powerful individuals."
Emberwyrms, raised by necromancy from red wyrms of the red dragonflight.
FrostWyrm Frost wyrms, raised by necromancy from wyrms of various dragonflight.
Magmawyrms, raised by necromancy from black wyrms of the black dragonflight.
http://www.wowwiki.com/Undead_dragon
Sylvanas has a few powerful forsaken necromancer's within her ranks that would certainly be capable of raising lesser dead dragon's.
If she's smart she'd wait until she could amass a quality number of them before revealing them to her enemies.