Harry should be able to get Dragon Nail in the D2:ED section of the game, as well.
If Ygerna opened a portal to escape to the Plane of Hypnerotomachia, or a portal was created when the explosion hit as she was teleporting, you could have just followed her in. Zandalor or Bellegar could have teleported you there, though Zandalor probably would have came with you if he had recovered enough, and Bellegar isn't usually inclined to offer help.
It is strange there was no cutscene or even a reference in the opening conversation explaining how you got there.
