It seems to me that the only difference between an exit poll and a voluntary disclosure checkbox which, if utilized by a statistically significant portion of the electorate, could be leveraged by official government commissions to audit election results where online voting is offered is that one these options is in the price range of the national parties and not publicly disclosed and the other strikes fear into the heart of everyone I've spoken to involved in election process.
Voluntary disclosure doesn't have to be a slippery slope and I firmly believe can and will lead to more fair and equitable access to the voting process and greater participation if it enabled online voting to be audited.
No one believes that large numbers of people will be blackmailed into disclosing their vote and if this or discrimination happens at scale then make it explicitly illegal. For those who aren't subject to these factors and are willing to participate in a census it would completely change how elections are run for the better.