Interesting, thanks. In pulling documents from EDGAR and other sources, there seem to be a few other alternatives, including several that explicitly state that the trustee will deliver all withholdings back to the employer for remittance and reporting.
My overall reaction is that splitting up the reporting, remittance, local tax (if applicable), FICA, 941 info, and multiple W-2s is convoluted. It would seem easier to have the employer run payment through its payroll, show proof of payment to the trustee, and request reimbursement from the trust. The few examples I've found explicitly permitting that approach are from the early-to-mid 2000s so maybe preferences have changed?
Alternatively, this article ran in Spring 2020 and suggests the reimbursement approach should be acceptable, even if not explicitly addressed in the IRS model document:
https://www.thompsoncoburn.com/insights/publications/item/2020-04-27/rabbi-trusts-taxation-basics-and-drafting-beyond-the-model-language
Either way, it sounds like the ultimate answer is that there's not one clear answer. Appreciate your input and time.