The CSI lines are high-speed connections, and there are several of them. So patching them out from test points is not exactly trivial.
In addition, the cameras have I2C control. On the Nano dev board the pin CAM_MUX_SEL / GPIO06 controls how the shared I2C bus CAM_I2C_* is multiplexed onto the used J13 connector and an unconnected camera slot. I haven’t yet checked the code to see if CAM_MUX_SEL is actually used, or if the multiplexer has static configuration.
In order to use three or more cameras, the control of CAM_MUX_SEL must be done by additional GPIO pins, or more I2C buses must be assigned to the cameras.
Designing a breakout board for I2C mux and camera connectors is doable, but still requires quite the effort. With the hardware in place, the software stack must be set up to work with multiple cameras as well.