That would be badass as fuck. I expected Kaido or big mom to do that with sheer overwhelming willpower/haki etc but alas.
Someone's going to break Seastone in this series.
I think Mihawk and Shanks can cut it, but that's just wild speculation.
Black Blades have to have some power beyond just being really strong swords: what if that's it?
A Black Blade channeling Haki from a Swordsmaster can break Seastone using Conqueror's Haki, and Armament Haki, with Ryuo.
I used to think of Ryuo as an application of Armament, and I no longer believe that. I think Ryuo is when you insert your Haki directly into another being.
So, Imu's Domi Reversi would be Ryuo based, since they're forcing their Supreme King Haki into others to Dominate them. Regular CoC just knocks you out; the most Advanced Forms are pretty much the Imperius Curse from Harry Potter.
Future Sight is Ryou based; you're sensing your opponents intent. This is why Shanks can cancel Observation; he's using his own Haki, to stop others from using Ryuo to push their Observation Haki into them.
If Doffy fully Awakens his Supreme King Haki... I could see him overpowering the Seastone.
Another possibility, though: we've seen Doffy create artificial limbs, via his clone, like Robin. Shiki escaped by cutting of his legs. We also know Doffy can use his Strings to mitigate bleeding and sew wounds up.
There's a legit chance Doflamingo cuts his own limbs off to escape.
"Damn it! Someone broke into Doflamigo's cell, and cut his limbs off! Quick, get in there, Magellan... er, Hanyabal will chew us out, if he dies! Quick, get him out of those chains, so we can move him!"
Doflamingo smiles. "My limbs are a small price to pay... for the destruction of this disgusting world."
Shiki already gave us the game plan, it's dismemberment.
Doffy with string limbs is kind of terrifying.