Tracking remaining charge in a battery is a spotty business... A percentage would be similarly inaccurate as the bars, just slightly less so.
It's sort of like tracking GSM 'connectivity' with bars, very unreliable stuff.
I wish at least that there'd be a full cfw which had mildly more accurate features as such, but alas... That's not likely to happen, and this 'homebrew' would be like an application, and would only be of use when executed. Which means you'll only see anything while it is loaded.
Don't get me wrong, as an idea, it's not crazy and it's not a bad idea. It's just not hugely possible, right now anyway.