In order for you to make a mega back door ROTH, you'd need to be able to make after tax contributions to the 401K plan. The option to contribute in after tax dollars is not available with CVS future fund. CVS future fund only allows ROTH and traditional.
I've done a couple of those when I was with Target but frankly, I don't think it's worth the risk. One IRS audit and one stubborn auditor would mandate you to liquidate the ROTH and pay penalties/taxes on the cumulative gains. While you may win at court, it will probably cost more in legal fees than the tax (along with penalties) that you'd be paying.