Narset trigger ability

[An Angry Fatman]

can I cast an overloaded {mizzium mortars} with {narset, enlightened master}'s ability?

[Remillo]

First, can a mod from this area scoot this to rules?

Nope.  Both 'Not paying it's mana cost' and Overload are alternate costs, and you can only choose to pay one alternate cost for a spell.