Please note that we carry 2 products that have “G2 18v25” in the name. These are the High-Power Simple Motor Controller G2 18v25 and the G2 High-Power Motor Driver 18v25. In this case, since you mention serial signals, it sounds like you are referring to the Simple Motor Controller version (SMC G2).
You can use the SMC G2’s serial interface to read the voltage on the analog input pin; however, I am not sure how that relates to the 78Hz duty-cycle based PWM signal you described, since that is not an analog signal. The SMC G2 does not support that kind of PWM signal as an input.
The previously mentioned G2 High-Power Motor Driver 18v25 does accept this kind of signal as an input, but in sign-magnitude mode it also needs a digital DIR signal. You could remove the need for a digital direction signal by using it in locked-antiphase mode, but 78Hz is a really low frequency for that kind of control. Typically, locked-antiphase control requires a high frequency PWM signal since it is essentially switching the direction of the motor constantly at that frequency. Also, please note that the G2 High-Power Motor Driver 18v25 is not as configurable as the SMC G2 and does not have many of the higher-level features like USB, acceleration limiting, input mapping, and much more.