jamesh: In the version of STV that I know, there are a couple of differences to that which you describe.
The main one is that surpluses are not distributed as you describe exactly. It would be more accurate to say that 80% of campd’s votes would be distributed at 100% strength. You cannot have partial ballots.
Another minor difference is that if a candidate gets pushed over the quota by someone else’s surplus or elimination, then only the votes which pushed them over the top get examined for surplus redistribution. This is a pretty controversial practice, which almost certainly changed the result of at least one constituency in Ireland last time around (Dublin North Central, IIRC).
Correction: It was Dublin Central. When Joe Costello was elected by transfers from Jim Mitchell, the surplus distribution favoured Dermot Fitzpatrick, even though the total vote for Costello would have favoured the more left-leaning Keho. The fact that the surplus came from Mitchell, a center-right candidate, skewed the next preference down.
There is also a GPL application to handle STV counts, which would make the task easier: pSTV. So perhaps the idea of having people do the 2004/5 election using the existing system, but specifying preferences as a test-run of STV, would be an interesting experiment.
