Not FU, Ancillery. Building a faker circuit is never the way you want to go on this sort of thing if it can at all be avoided.
First, the execution of the circuit would require that you know exactly how the system works under all conditions so that you can fake out all conditions. This would likely require a custom circuit board that can receive a piggybacked signal from various sensors along with a program that makes a decision based on the sensor feedback so that it can send out the correct fake signal.
Second, such a feedback setup would likely mask any electrical problems until they became severe enough to affect the feedback circuit itself. Since the feedback circuit would draw so little power it would likely work with a battery too dead to run more than the radio, you'd not find out about an electrical problem until you were walking home.
Third, if you took the car into a shop to find out why some random system isn't working right and forgot about the system being on there, or didn't connect the possibility that the failure would be due to a low voltage condition, it would wind up costing a mint to fix. On a lot of problems one of the first things to be checked would be voltage, and most people would see the gauge moving correctly and see that the computer is reporting back that the electrical system is working perfectly when it wasn't. It wouldn't be until deep into the troubleshooting, after multiple unnecessary parts were replaced, that someone would start going back over the troubleshooting steps and thinking that perhaps the computer needs to be verified. When they see the computer reporting no electrical problems but test gear showing low voltage, they are going to assume that the computer is also fried.
If you build one yourself it'll take months of testing to get everything working correctly, and if you hire the work out the person you hire will want to be paid for those months of testing. Either way, it'll cost more than just getting the programming fixed and none of that will take into account the extra expense of false data causing repair problems for seemingly unrelated problems.