Thanks for the responses. My 401k plan allows after-tax contributions, maintains them in a segregated subaccount, and permits in-service withdrawals of the same up to twice per year, but does not permit in-plan rollovers/conversions. So the only way to get the after-tax subaccount balance into a Roth account is to take a withdrawal directly into a private Roth. The IRS guidance cited above deals only with in-plan rollovers/conversions. It sounds like there may still be some doubt as to whether the pro rata rule is a problem for direct withdrawals into private Roth (both with respect to the 401k subaccounts not being withdrawn and/or with respect to any private traditional IRA accounts already in existence). Anyone disagrees with this summary of the current state of affairs?
There is definitely controversy on the pro-rata rule. Your task, as an investor, is to determine if one side is comprised entirely of idiots. The presence of controversy does not mean unknowability. When you're researching this for yourself, be sure to continue to draw the distinction between IRA rollovers and in-plan conversions. Also be sure to draw the distinction between people who are saying that IRA rollovers trigger pro-rata tax
within the entire plan versus
within the after-tax subaccount.. There are lots of credible people making good claims that the latter is the case, and I believe them, but it's easy to confuse their arguments for the other thing. (There are also people claiming that you can do clever tricks to completely isolate your basis within the AT subaccount, and wind up with all your basis in a Roth IRA and your gains in a tIRA, and wind up owing no tax whatsoever on the conversion. I don't buy that, but some fiduciaries will let you try it.)
I think Trirod sort of nailed it, except it's actually really easy to figure out if there are segregated subaccounts (as you have clearly determined), and I suspect most major fiduciaries do this by now. Ask, for sure.
If segregated subaccounts are allowed under the law and your plan provides them, and if pro-rata treatment is still required on withdrawals, then what the hell does a segregated subaccount mean? I haven't seen a compelling answer to this question from anyone saying this is unpossible. Most seem to just throw up their hands and scream "Controversy! Uncertainty! Run!"
I think you're right that the IRS guidance is more clearly in favor of in-plan conversions than IRA rollovers. But I think the case for IRA rollovers is also pretty strong, and you have a lot more security in numbers there. Almost no one does in-plan conversions, but the IRA rollovers are very popular. It would be very hard for the IRS to suddenly come out and say the IRA rollovers are disallowed without grandfathering a lot of past behavior and giving plan administrators time to adjust.