[Coco] How do I program a new ECB ROM with minimal external hardware ?

[RETRO Innovations]

I don't want to squelch your desire to use the RPi, but if this is a
means to an end, you might want to consider this:

USD $13:
https://www.amazon.com/Programmer-Programmers-Sp200Se-Enhanced-Version/dp/B0C3NF4YMK?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A38BA2GPIHDS1B

Jim