I don't think that soul siphon does work on staves. For one, staves aren't, strictly speaking, enchanted weapons. They don't ever physically hit an enemy, how most enchantments are applied, but rather they cast a spell at their target. The spells is a separate entity in itself, and typically not tethered back to the staff. Now, the staff of magnus' absorb effect is tethered back, so that one might work, but others won't, and the staff of magnus probably doesn't either.
I don't think that soul siphon does work on staves. For one, staves aren't, strictly speaking, enchanted weapons. They don't ever physically hit an enemy, how most enchantments are applied, but rather they cast a spell at their target. The spells is a separate entity in itself, and typically not tethered back to the staff. Now, the staff of magnus' absorb effect is tethered back, so that one might work, but others won't, and the staff of magnus probably doesn't either.