Since Shen's skills scale with AP and very well for that matter I'm surprised that nearly no one buys AP items for him. What is the reason for that? There are a lot of semi-defensive ap items or ap items with health and armor.
Somewhat viable to a certain extent, but not to the extent of getting Rabadons, for instance. He gets so much more from stacking HP and resistances, since he is, in essence, a damage-soaking tank. His kit (a shield, a taunt and a global shield-teleport) make it a must to build tanky.
Pro-tip though: (This is not for Shen jungle, but for Shen Top.) Build a rylai's crystal scepter if you get ahead, which you should, since you are Shen after all. That bonus AP and Health are great for Shen, and the 35% slow on Vorpal Blade is just too much. That slow + taunt and you'll shut-down the enemy carries every time.
I think it's viable, but it ends at Rylai's and masteries 9/21/0 going for the MPen.