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using the input coin itself as the identity

I don't see how that would help countering DoS, you won't likely be reusing inputs so there can't be reputation tracking.

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A specialized for doing a meet in the middle auction (e.g. a few comparison operations and an arithmetic average) is a whole other of magnitude less complex than running a complete transaction processing program in it. Going and figuring out where the state of the art is in that space has been on my todo list.

It was a bit more than that IIRC. It was highest bid wins, with winner paying the price set by the second highest bid, and it was NOT just two or three participants, it was a LARGE auction. A very large amount of offered goods and bids.

Besides for risk of sybil+correlation attacks (which can be partially countered through reputation tracking), I like my SMPC scheme.