i2c bus sniffing and spoofing
Of course it's possible, at worst by cutting the wires to the original peripheral and interposing something with a controllable pass-gate, or perhaps even just a strong push-pull driver able to overpower the original peripheral.

This (perhaps without disconnecting the original) is for example what the "Bus Pirate" series of debug boards was designed to do.

It would also be very possible to make something with some other MCU controlling pass gates, or a small FPGA alternately passing through or overriding the peripheral side of the bus.

Unless your messages have end-to-end encryption and cryptographic validation between chips you have to consider that they are intercept-able and spoof-able.  If someone is sufficiently determined and budgeted, even on-chip signals could be.  And without a lot of care, the behavior of the peripheral chip could turn out to be fully emulatable.