Doesn't exist. All the "hacked" firmware is for the Broadcom chipset used in the Linux based routers such as the WRT54G.
However, if your purpose is to extend the range, it's much more productive to do it with antennas than by cranking up the power. Let's say your dream is fullfilled and you find a way to increase the TX power of your BEFW11S4 from +15dBm to +25dBm. That's a 10dB increase which is about what you get with a biquad, coffee can, or panel antenna. However, it's only benificial in transmit. The world can hear you 10dB better than before, but you can't hear any better than before you hacked the TX power. An improved antenna increases both the TX and RX range. For every dB of improvement in transmit, you get the same improvement in receive. Therefore, you're range increases symmetrically. Look for a better antenna.
Incidentally, there's a name for such devices. They're called "alligators" which is an animal with a big mouth (TX) and small ears (RX).