Ok, so coin age would be your DoS countermeasure?

Note that attackers don't actually have to spend that input if they disrupt the protocol. They can use the same one over and over, even many times simultaneously.

PoW could be an option where participants can set a minimum PoW accepted and a maximum PoW they're themselves willing to perform. So for example if 10 people all have 2^15 work within their range of acceptable PoW (one has 2^15 as their lowest, another as their highest, they'll all generate a common input for PoW and start working on it. Then they start this scheme with the SMPC. That would indeed make the attack costly for somebody trying to break the scheme for as many as possible (DoS) and would make Sybil very costly.