In a number of professional practices that are P.A.'s (C corporations) employment contracts often provide that the individual will receive a specified percentage of any amounts that are collected for that employee's services after the employee terminates employment. Identical provisions will be in the contracts of all professionals.

Does anyone treat these provisions as ERISA severance plans?

There are some old cases which state that amounts payable under an employment contract are not deferred compensation payable under a plan subject to ERISA because the amounts are indivdually negotiated. Most employment contracts for executives provide for some payment after termination of employment but are not considered ERISA plans because the payments are made in return for non competition, confidentiality promises or consulting service to be provided by the executive.

The more interesting question is whether the person can make contributions to a section 125 or 401(k) plan based on those post-termination payments.

I don't think they they can. With respect to 401(k) deferrals, the IRS has generally taken the position that only current employees (not former employees) can make deferrals to a 401(k) plan. I haven't looked at the issue with respect to 125 plans, but if contributions can only be made by current employees, the result would be the same.