The main use case was to put $ into your own 529 while working and then move it to your own Roth. I left out the while working part and see how that could cause confusion.
I agree with EvenSteven. People are trying to be clever and take advantage of this new law in ways not intended - a practice which I applaud in general. But the way it's written, there's really not much opportunity to be clever with it.
If you're semi-retired and have enough income to contribute to a Roth, then just contribute to the Roth then, in semi-retirement.
Doing it 15 years ahead of time via a 529 would start the tax-free treatment sooner. But you have another account to manage and keep track of. If the money is destined for retirement/Roth, you could just put it in your 401(k) or do a backdoor Roth. If you have so much excess income that you want to go into retirement and have maxxed everything else out, then putting it in taxable in an index fund and spending the dividends is nearly the same thing and a lot simpler. And a taxable account typically will have lower expense ratios than a 529 plan - in my state it would be about 0.25% per year - over 15 years that would add up.
The other cleverness with the 529 idea here is that you might get a state tax benefit with the 529 contribution. That might be helpful.
If you do this, it's limited to $35K total per beneficiary per lifetime. If your accumulated contributions plus investments end up above that, then you'd have to figure out a plan for what to do with the excess - take it out and pay taxes/penalties, give it away to a relative going to college, or something else.