Assume a Participant does not have a designated beneficiary. If he dies prior to his required beginning date his entire interest must be distributed under the 5-year rule. What if he dies after his required beginning date? Does the 5 year rule apply there as well?

no

So pay it out at least as rapidly as it was being paid out (assuming the plan doc doesn't require it to be paid out sooner)?

Regulations require that the benefit be paid out based on the non-recalculated life expectancy of the deceased account owner as of the year of death. 1.401(a)(9)-5 Q & A 5 (c)(3)

Annotated Regulations



Here

In the absence of a signed beneficiary designation form, remember that there is a designated beneficiary based on the plan's provisions that are applicable when there is no beneficiary designation form. (Treas. Reg. Section 1.401(a)(9)-4, Q&A-2.)

Agreed. But if it turns out that the estate becomes the beneficiary then my previous post applies.

Correct.