Sorry, I dot have the error message anymore but after downloading more than 2GB of data into a temporary file lynx spits out an error message that it cannot write the file. I do not see a largefile USE flag or similar to enable. However, I see --disable-largefile flag for configure. Please test yourself with: "lynx http://gentoo.chem.wisc.edu/gentoo//releases/amd64/10.1/livedvd-amd64-multilib-10.1.iso" I do not have the bandwidth.
So far, the only way I managed to get lynx to refuse to save a downloaded file was to run out of disk space, but I assume you already verified there is plenty of space to save the file... Also, the ebuild is not disabling largefile support, and my build log agrees. Please post your 'emerge --info' output and attach your lynx build log to help me try to reproduce the problem.
This bug has been sitting for quite a while... have you stopped running lynx and lost interest in this bug? If not, could you please attach a build log & emerge info? Otherwise I'll have to close as 'worksforme' because I couldn't reproduce it yet.