I'm developing on a system that's running Lucid 10.04.4 LTS. I can't do a complete release upgrade in order to get access to newer packages, although I do have sudo so I can otherwise muck about as needed, as long as I don't change the release version or kernel.
With Lucid's repos, including backports, I can only get boost 1.40 packages. I need a feature from a newer version of boost (I think - I'm trying to find a workaround in parallel with this questions), but I'd really like to avoid installing from source because that can turn into a rabbit hole... especially since newer packages do exist; just not in the standard Lucid repos. Is there a repo somewhere I could connect to and get newer boost packages? (Basically, I'm looking for a backport, but one that doesn't appear in the official Lucid backports...)
You are looking for backports that don't exist in your upstream repository (because of the end of life thing). So the answer is that you need to:
The former means adding something to your /etc/apt/sources.list file manually or using add-apt-repository. I can't recommend a given repo since I don't know what you need, but here's an example for nginx:
http://wiki.nginx.org/Install#Ubuntu_PPA
You should look for similar repos for whatever software you want.
Fair warning: your expectation that it will be easier to not change the base Ubuntu version is questionable. At some point you will have cherry picked a whole bunch of components that were designed, tested and documented primarily against newer releases and it would just make sense to be on that release.
For development purposes you might find it way cleaner to just install/run VirtualBox (or another virtualization platform) and put the latest and greatest OS on there virtually.