Roth 401k and after tax are actually 2 different things. Roth 401k is after tax money but not technically categorized as such. If you wanna do MBD, you need to contribute to "after tax", not "Roth 401k." At least for me with Fidelity and my workplace plan. Ymmv

I convert every 10k ish from after tax 401k to my Fidelity Roth ira. I'm calling in to do that about every 2 months now. Not that bad. I just changed my contributions to a stable value fund so there won't be any gains to pay tax on for the conversion. Yeah I'm missing out on a bit of gains, but 2 months isn't much anyway on working up to 10k