Hey all! I've been reading for awhile but never posted. I do have the army book with me though, so I thought I'd provide the applicable information about Channeling.
Channeling:
A Mage-Priest has the ability to take over the mind of a Skink Priest in order to cast a spell through its minion.
The Slann can channel a magic missile spell through any Skink Priest within 24". The spell is cast as if the Slann model was where the Skink Priest model is. Any miscasts affect the Slann as normal.
The Skink can cast his own spells in the Magic phase using his own power dice.
Based on that alone, I'd say the channel wont work if the Skink Priest is in close combat.
In the Mage-Priest ability description is states:
Guardians:
When joined to a unit of Temple Guard, a Slann is always placed in the second rank. A Slann placed in the second rank may still act as normal (he may cast spells, use magic items, act as a general, army banner bearer etc). The Slann Mage-Priest's palanquin allows it to float up to cast its powerful magic before dropping back within the protection of its unit. Because of this, his line of sight is worked out as if the Mage-Priest himself was a large target. In addition, as long as the Slann has no enemy models in base-to-base contact, it may still cast magic missiles even if his unit is engaged in close combat. If the Slann joins any unit other than Temple Guard, it is placed in the front rank as normal.
(I fixed the MANY typos in the book. /slap to the editor.)
I'd say that continues to support the theory that a Skink Priest on a Stegadon that is in close combat would not be able to channel the magic missiles.
Any other readings?