There's no reason in principle that a lily (of any of the types you mention) can't be planted on a slope since they like good drainage and their bulbs should be planted relatively deep (~8" if mature) and are well rooted once established. It seems like you have plenty of sun for them, though they would benefit from a ground cover to keep their "feet cool". Lilies are said to like "their head in the sun and feet in the shade".

But, if you are asking if lilies are noted for being good for erosion control on a steep bank I'd have to say probably not. (Daylilies are used for that.)

-Helen