tl;dr 99.9% sure we can actually cast Ancestral for free off this immediately.
Former judge experience would dictate that I'd wait for the FAQ before anything. The card says to pay 0 rather than it's mana cost. Depending on the rules, you're either replacing a mana cost of "no mana cost" with 0, or you can't replace that.
It's a weird corner case that needs a ruling. That said, rules citation: (emphasis mine)
601.2b If the spell is modal, the player announces the mode choice (see rule 700.2). If the player wishes to splice any cards onto the spell (see rule 702.46), he or she reveals those cards in his or her hand. If the spell has alternative or additional costs that will be paid as its being cast such as buyback or kicker costs (see rules 117.8 and 117.9), the player announces his or her intentions to pay any or all of those costs (see rule 601.2f). (irrelevant stuff afterwards)
117.6. Some mana costs contain no mana symbols. This represents an unpayable cost. An ability can also have an unpayable cost if its cost is based on the mana cost of an object with no mana cost. Attempting to cast a spell or activate an ability that has an unpayable cost is a legal action. However, attempting to pay an unpayable cost is an illegal action.
To get to the point: Attempting to cast Ancestral out of your hand without suspending it is legal. It only becomes illegal when you try to pay an unpayable cost (namely, the fact it has no mana cost). Given this is an alternate cost, we are paying that rather than this, so we are legally in the clear.
