Keep in mind that by the time a recordkeeper sees a deferral, that deferral already has been processed by payroll. A recordkeeper can only react to whether payroll has applied the catch-up rule correctly.
Generally, payroll for self-employed individuals is run separately from payroll for common law employees. The self-employed payroll commonly pays a draw and processes some deductions for fringe benefits or insurance, but does not apply payroll taxes. The self-employed individuals are responsible for making estimated tax payments directly with the IRS or through their tax accountants.
It is not uncommon for a payroll interface file sent to the recordkeeper to combine the two separate payroll runs into a single file.
If either payroll or the recordkeeper keeps an indicator for self-employed individuals, the indicator will need to be able more sophisticated than just a "yes/no" indicator. For example, the recordkeeper will need to be able to discern when an employee changed between being a common law employee and a self-employed employee in the prior year, or need to know how much compensation in the prior was FICA wages and how much was self-employment income.
Maintenance of an indicator likely will fall on the employer, regardless whether the indicator is in the payroll system or the recordkeeping system.
I expect when these provisions go live, a full-contact game of hot potato will break out between payroll and the recordkeeper with each side saying "it's your job, not mine."