Instead of posting a passworded wallet and then when it is time to launch the coin the dev posts the password; The dev should have the wallet only let the users mine after launch, excluding the genesis block (obviously). This will stop instamining very well because everyone has a chance at the first blocks. Plus everyone will know that the dev didn't instamine before everyone else.
It's a bit like what the GOODcoin Devs have done with their coin when they ran into problems.
https://bitcointalk.org/index.php?topic=578963.680(They have it so that everyone can update their wallet before mining starts (in 2-3 days) so there are no forks)
