This should probably be prioritized fairly low, but it's possible that it would be very easy to implement if the hardware supports it. I am wondering if it would be possible to incorporate a readout of the input voltage as one of the "instrument" options.
Two things have made me think of this. First, I've been playing with an external battery power pack for the iFly, and a readout of the voltage might help me gauge battery health and/or state of charge, and over time I might develop a feel for how much runtime I could expect.
Also, in reading about a case of alternator failure during a flight on another forum, someone posted that they use their Garmin handheld to monitor ship's power system voltage while it's plugged into the cigarette lighter for additional insight. My aircraft only has an ammeter, not a voltmeter; noticing a change in ship's voltage via the iFly could provide early indications of developing electrical problems before other symptoms present themselves.
Now, no one buys a GPS so that they can monitor voltages, so I'm not going to argue that this should jump to the top of the development priority list. However, if there happens to be an input voltage sensor inside your gizmo, it might be a relatively minor effort to add the capability, and so I thought I'd ask you to consider it.
Thank you,
Jim