adding fluctuations to 5V
I'm assuming the following


You have control over hardware and firmware of the sub-components A and B.
Each location L and R has a connector (or a set of connectors), which provide: power, ground, connection to I2C


Then there are several options, which revolve around adding a location pin to the location's connector.  This pin can have a digital signal (low for Left, high for Right).  Or, it could have an analog voltage set by a fixed voltage divider (the voltage would be distinct at each location).


A microcontroller (μC) in Circuit A (and B) can read its locating pin and choose its I2C slave address based on the location.
A μC can have a fixed I2C slave address.  The slave would report the value of its locating pin during system initialization.  Then the software on the master would take care of routing the messages.