wouldn't there need to be voice lines for all characters (including custom?)
Well ... yes. :-/
That can be problem indeed.
It could probably by bypassed by just letting Narator tell us something like "Gale send you his memories ..." and then play exactly the same soundfile ... but since Larian records every responces separately, i guess this wouldnt fit their profile.
Also it would feel pretty stupid.

Why would s/he use the tadpole
Oh you missunderstand me, i just wanted to point out that there is that option ...
Since i had in mind scenario, where Kithrak is trying to use Read Thoughts on us ... and we would really not want Gale to start yelling:
"Carefull Tav, he is trying to read your thoughts, steel your mind before they learn about our tadpoles!" 
(Just as Shadowheart do in exactly the same scene, where she talks right before them about that we should not help them.)
Since Larian would need to add those lines one-by-one ... they also should include logic that surrounds the scene.

I thought that is obvious.