Emulate Full Battery Charge
From what it sounds like, you not only want it to show a higher percentage but also 'use' this last little bit of juice in the battery.  Technically you could just modify the icon to show full all the time or be slightly above the real level, but that's just the icon's look.  I think to accomplish what you're trying to do you will have to modify the ROM yourself.  If your savvy in this field, you can look for some open source ROMs, modify it, and see if it works.