If the account is not vested, I think the nonvested portion of the RMD is added to the next years' RMD. I did not look this up, but that is my recollection.
So if the DB plan has a 3-year cliff schedule, uses the later of age 65 or 3rd anniversary of entry for NRD, and excludes years prior to the plan establishment for vesting purposes, then any RMDs for the over age 70.5 owner which are not vested are thus not paid but they are carried forward to increase the next years' RMD.
Correct me if I'm wrong, but that's my memory on this.
As to the solo-k topic here. If you're a TPA that tracks time spent by client, you might be surprised by how much hand-holding these owner-only plans actually need - especially in the first several years, and very much especially if they are also doing their own investing (no advisor - they're paying the TPA, so how can they afford an advisor too, right?), and even more so when it's time to end the plan. They can really eat up a lot of your time. Remember, they usually don't have staff to help them, so once they find you are reliable and straight forward about how to handle their plan, they call and ask you for all kinds of help, such as "How should I be filling out this investment form for the plan?", "Who do I write the check to?" "How does the distribution withholding work and how do I get that paid to the IRS?". And when a contribution deadline is approaching you might have to remind and explain multiple times and in various forms why you need the data request completed and returned. Perhaps it would be better for someone else to take a crack at these plans. Maybe I am wrong about this, but I am not convinced these plans truly help the bottom line of a TPA, other than it can build relationships that turn into additional future business. IMHO, FWIW.