Why not use one of the newer chips with a built-in bootloader such as Tem.. aaargh Atmel T89C51Rx2, Philips P89C51Rx2. Also I saw a product introduction from Dalls of a chip with built-in ISP. I do not know for sure about the others, but I have the Philips chip doing ISP with nothing but a special command through the command interface, then using FlashMagic (free from esacademy.com) to bootload through the 232 port. The datasheets and application notes will have you believe that you need switches and gates, but - at least for Philips - that is not the case.

Ha' en herlig dag

Erik