I haven't found a john the ripper implementation for ECDSA, or it doesn't exist.
From the code you pasted, it seems that the real issue here is the 10000 rounds of SHA256, not the ECDSA. It may be worth trying to put only the "stretch_key" method on a gpu, and then do the ECDSA on the CPU. Probably much less work to implement, but still a significant speed-up compared to naively running this python script.
A friend of mine said that he tried but he can't run ecdsa at a good speed... Not sure of why this doesent agree with other posts here...