Instead of posting a passworded wallet and then when it is time to launch the coin the dev posts the password, the coin dev should have the wallet only let the users mine after the time of the launch. Except 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)
