I have Sedris, the Traitor King on the battlefield and a Grinning Ignus in my graveyard. I use the unearth ability Sedris grants to unearth my Grinning Ignus. Can I activate the Grinning Ignus's ability, even though if I try to pay the cost the Ignus will get exiled instead of going to my hand?
Can I do the same with an Ovinomancer, provided I activate its ability in response to its enters-the-battlefield trigger?
More generally, is there a rule that governs what happens when a replacement effect modifies the payment of a cost?