Dynamically controlling the EA line should work for most cases I know of as long as you subject the MCU to a reset after each desired state change of the EA line.

It is true, as you noted in your edited comment, that an acess to an address that goes higher than the extent of the interal flash would become an access to external memory --- AS LONG AS the MCU in question actuially has external BUS support. Many of today's MCUs do not have any external BUS support and in theses cases you could get several types of behaviors for accesses outside the internal memory space size. In some cases for these accesses will simply ignore the higher address bits aand the accesses will redundantly decode to the internal code space. On others accesses like that can lead to erratic of undefined behavior and could result in crash of the MCU till a reset.

Michael Karas