You seem to have already identified the two most obvious choices:
Electrical Engineering is the site to choose for questions about modifying hardware for custom uses. Although questions about household appliances (such as coffee makers) are generally off topic there, there's a specific exemption for questions about hacking them (emphasis mine):
"[I]f your question generally covers […] a specific electronics design problem […] and it is not about […] consumer electronics such as media players, cell phones or smart phones, except when designing these products or modifying their electronics for other uses […] then you're in the right place to ask your question!"
However, rather than modifying your coffee maker itself, you seem to be more interested in just reverse-engineering its barcode data format, so that you could design your own barcodes. This makes your specific question less suitable for electronics.SE, but a natural fit for Reverse Engineering, which explicitly accepts questions about "hardware analysis and testing".
Ps. If you want anyone here to be able to help you, be prepared to offer thorough examples of as many bar codes as you can, with corresponding product information. Also, sooner or later, you're presumably going to have to try feeding some modified bar codes to your coffee maker, so you might want to do a few quick experiments with that first and report the results. For that, assuming that it won't just error out, it'll probably help if you can measure as many objective properties of the brewing process (e.g. time consumed, amount of water used, water temperature, etc.) as you can.
Also, please note that, while it may seem very unlikely in this case, as with any hardware hacking, there's always a risk of breaking your hardware and voiding the warranty. Given that you're dealing with hot water under pressure, I'd even recommend grabbing a pair of safety goggles, just in case. While I'm about 99.9% sure your coffee maker won't explode just because you fed it a bogus bar code, it's still better to be safe than sorry...