"If I do a direct rollover of the Roth 457 to the Roth IRA, can I then withdraw the post-tax contributions I made to both accounts tax and penalty-free?"
Yes. Your contribution basis in the Roth 457(b) becomes contribution basis in the Roth IRA. The Roth IRA ordering rules apply to Roth IRA distributions no matter how the funds came to be in the Roth IRA. This is one reason that people roll funds from a designated Roth account in an employer plan over to a Roth IRA, to be able to access all of their contribution basis without the distributions being pro-rated between nontaxable and taxable amounts.
