Before starting: You really shouldn't remaster Lucid. It's more than 3 years old, will cease support in April 2014 and most of the common packages doesn't receive updates. Remaster 12.04 if you must, but don't use Lucid. That said, my instructions may not work for anything before Precise.
Is there a way to download the latest versions of its packages from the repository without editing sources.list?
You can ask apt-get to use another sources.list file. Just add -o Dir::Etc::SourceList=/some/file
to the apt-get
call. This may work for aptitude too.