Fused Staff states, in part: "You can Cast Spells from the staff as part of a Spellstrike even when the staff is in weapon form. Otherwise, you can't cast the staff's spells while it's in weapon form."
So it would work beautifully with a Starlit Span Magus. I've read that a Staff of Divination is a great choice for Magus due to it having True Strike on it, which significantly increases your chance to land those big spells for big damage.