If a battery fails when there is no-one to see it ...
> a pair of LEDs to give a visual indication
That would be a visual indication to whom, exactly?
I'm a great fan of flashing lights on computer boards (the microprocessor equivalent of Hello World) and I recognise that in the world of Pi, they are a massive sales feature. But you have to ask: if a board is going to be run off batteries, possibly even built into an enclosure, what is the point of some LEDs - once you've debuggered everything? Given their power consumption, it would be nice if they could be pulled off the board, or just given the SNIP.
If the developer is looking for some extra features, if the on-Pi thingy doesn't already do this, I'd suggest having an ADC measuring the state of charge and use that to talk to a serial: RS232, SPI or I2C interface (as the Pi doesn't have any analog capability of its own) to inform the board what is going on. In fact, breaking out a few 10-12 bit analog inputs and having them interface to the Pi would make this a winner: even without all the battery stuff.
As it is, there are already other (yes folks: there are other single board computers) SBCs that can directly plug in a LiPo without the need for an extra board. Some are even open-source hardware.