hi!
I think what you want can be done!
1, Connect EA pin with any port pin. when reset is happen. You will automatically be in internal ROM.
2, map external memory same as internal memory. i.e. map it at 0000 Hex.
3, Put following type of Code after interrupt table and before your main program.
a, make sure that all outputs used to communicate with External Memory will be inputs (i.e. FFH).
b, change EA pin to GND.
c, your first program instruction in external memory should at then next address where above 'clr EA' instruction is situated.
d, when going back to internal rom follow the same rule except you internal ROM should handle mcu after 'setb EA' instruction of external rom.

I think above is the way to switch between internal and external memory. It is similar like a bank switch, but with some precautions.
Finally, A good advise, that is, if you don't have good understanding and experience in 8051. Dont try to implement above technique or your brain will suffer very much ;-).