If you write a small bootloader and von Neumann-wire an EEPROM or two that supports RAM-style writing in system (such as an AT28C64B or a AT2864E), you could achieve in-system programmability without any form of clock gating or other complexities.

if he is to get another chip to avoid "any form of clock gating or other complexities", then, by Jove, it ought to be an ISP micro, not another memory chip to promulgate the workarounds for "this is what I can do with the chip I have".

Erik