Just throwing it on the table. This would br for a 5V write flash.

a) Make a(n electronic) switch that
1) switches PSEN to a boot ROM
2) switches RD and WR to the flash
3) disconnect any external RAM CE

b) make a piece of PC software that would transmit e.g. 128 bytes and then wait for confirmation.

c) the code in boot ROM would
1) read the 128 bytes
2) put them to the flash holding WR longer by a clr p3.6
3) read them from flash (now data memory) to verify
4) transmit accept/reject

Just ideas to get started, all responsibility for functionality declined.

Have a memorable weekend

Erik