I had a discussion with a representative of a trade association on this topic today who pointed to the 2022 Greenbook which gives us some insight into how Treasury was thinking about this a year ago. You can access that at the following link (page 106):
https://home.treasury.gov/system/files/131/General-Explanations-FY2023.pdf
Their proposals included, in part:
Section 7701 be amended to define on-demand pay arrangements
Section 3401(b) be amended to provide that ODP arrangements be treated as weekly payrolls
Sections 3102, 3111, and 3301 be amended to clarify that ODP arrangements are not loans
It's not specific to plan issues, but it's better than nothing.
It may also be a prudent practice for the client to document the position they take with regard to the treatment of on-demand pay, and rationale for same, to show that fiduciaries recognized the challenges this creates for their plan and made a reasonable decision that they can consistently apply, absent future regulatory guidance.