I2C or uart would require an additional wire, so that'd be cost-prohibitive. But I do agree that something like One-Wire would be better from a hobbyist perspective.
Although probably easy to implement in silicon, USB PD is an absolute nightmare to DIY. Pretty much your only option is to get a dedicated PHY like the FUSB302B to do the actual electronic part of PD comms for you.
Seems like it must not be that easy to implement, or it would already by in all the chips the way CANBus seems to be integrated in tons of stuff.
One-wire stuff is so cheap, and and they have EEPROMS meant for exactly this kind of electronic marking(I believe it's what power adapters did before USB), and it can be done with pure software on an MCU costing pennies.
Although probably easy to implement in silicon, USB PD is an absolute nightmare to DIY. Pretty much your only option is to get a dedicated PHY like the FUSB302B to do the actual electronic part of PD comms for you.