First thing to do would be apply the merged-mining patches, as bitcoin is only equipped so far to be the primary chain in a merge, not one of the secondary chains.
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Hmmm....merged mining is a thought.
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Another possibility would be a if simple third-party block black-list mechanism was in place. This would allow security with very low mining effort.
This would only work mainly under the conception that this 'legacy' line were pretty much completely mothballed and operated unprofitably by enthusiasts.
The idea (and hope) would be that Bitcoin proper thrives and none of the concerns that some of us have come to pass. Thus, the 'legacy' chain is never really needed.
I could see a situation where all mining proceeds were issued to current value holders (rather than going to miners.)
Of course if the worst happened and 'Bitcoin proper' failed in some way bringing 'legacy' back on-line, a lot of mining power would be available to secure the system in the normal (and vastly preferable) way.
In the mean time 'legacy' could be experimented with to develop methods of covert messaging, efficiency optimizations, and support for more capable secondary systems (a-la the 'Fidelity Bank' idea.) A switch between 'cheating' and 'real' proof-of-work should be pretty invisible.