One method would be to have an external latch 'a' which is reset on power-up. This latch can be (re)set by a port pin. On reset transfer the output of latch 'a' to latch 'b' and let latch 'b' control the memory swap.

I do not see the use of RAMloaded programs as an application for the '51 it is a microCONTROLLER, not a general purpose computer. If you are using this as a development method, why not just go for an IAP chip such as the 89C51Rx2 (Philips/Atmel)
I believe many boardmakes have this, Phytec for one has a very good development board with all the bells and whistles for 232 download to flash.

Have fun,

Erik