Under the channeling rule for skink priests, where the Slann can use the LoS of the priest to cast magic missiles, read the line carefully. "The spell is cast as if the Slann model was where the skink priest model is" Does this mean that if it is counted basically as the Slann being there, you can use his 'LoS as if he is a large target' special rule, so the priest may channel magic through the Slann to targets that even he can't see?
"The spell is cast as if the Slann model was where the Skink Priest model is." In other words, not on his palanquin floating up allowing it to have vision like a large target.
You cannot have a Slann without a palanquin though, no Slann has touched the bare tainted earth in over 1000 years, it would ground their magic. However, it is definitely obvious that it should be straight out of the skinks eyes. It is just from the wording, you could assume the Slann model is there so it can see as if it is actually there. Well, that is exactly what the rule says.
I think you missed my point. He is of course on his palanquin where he physically is but when seeing through the eyes of the Skink he doesnt have the option of floating up on his palanquin to see over the top of units. He can only see what the Skink would see and that is non large target stuff.