I've been Googling on and off for about a week, and I've come to the conclusion that no-one has ever managed to successfully compile firmware from source and install it onto a ADSL router. There's plenty for a "normal" router, but not an ADSL router, i.e. one that allows you to connect to your ISP.
I've tried it myself, downloading the ASUS source. Because of GPL, they're obliged to provide it, but they make it as difficult (well, impossible I suppose) to actually compile it due to all the extra libraries, specific toolchains and config files that are required. And then there's the problem of the hardware being pinickity about allowing custom firmware.
There's OpenWRT and DD-WRT. OpenWRT doesn't have anything for an ADSL router, and DD-WRT only seems to have it for one model, but that doesn't seem to compile and there seems to be a question of whether it is open source any more.
Part of the problem seems to stem from all the different chipset and hardware configurations, but surely someone somewhere has managed it?