And this time around I went with the suggestion made earlier in this thread of Amy Acker as T'Prynn, because I'm more familiar with and fond of her than I am of Bridget Moynihan, and I felt it worked pretty well.
I like this bit of casting. Like you, Moynihan never really fit in my imagination as T'Prynn. I still have a book and a half to go in my Vanguard reading; I think I'll plug Miss Acker in there and see how she works out. "Tonight, the role of T'Prynn will be played by..."
And since I'm already in this thread, I may as well put forth some of my own inner casting.
Mackenzie Calhoun: David Duchovny
Shar: Tyler Posey
Prynn: Keiko Agena
I've been reading New Frontier since the beginning and the former Fox Mulder has always been my go-to portrayer of Calhoun, though I can definitely see why a lot of posters seem to opt for Flannigan.
My reading of the DS9 relaunch started only last month so I certainly would have gone for another actor had I read these novels when they originally came out, but since I only began recently, I cast a "current" actor. On Teen Wolf, Mr. Posey has demonstrated a calm reserve for most situations and bursts of sudden action when called for, much like Shar in the stories that I've read so far.
Prynn's dual attitude (one for Vaughn, another for the rest of the crew) reminds me of Lane Kim from Gilmore Girls and her own dual attitudes with friends and then her mother...so I picture Agena.