Assuming the plan permits it, is there any reason why a 401(k) plan could not accept a rollover contribution (an eligible rollover distribution from the plan of a subsequent employer) by a former employee who is still a participant?

I don't see why not, but why would the plan want to accept rollovers from former participants? To build assets for economies of investment scale?

Private letter ruling 8816050 interpreted "employee" in Code Section 402's rollover rules to include an retired employee. It's not a very strong or recent piece of authority, but supports a conclusion that you legally could do this for former employees.