Are there any distros that are designed to make a wide range of very recent (unstable) packages available on low-RAM systems?
This kind of question gets asked a lot and it hinges on a fundamental confusion.
The amount of RAM you use is determined by the software you are running, not the distro you installed. "But", you say, "Doesn't the distro I installed determine the software I am running?"
Yes, but in the sense that (for example), ubuntu uses a different init daemon than fedora. I'm not sure which one of those uses the most RAM, but if you have a look at
top -p 1 (the pid of the init daemon is always 1) you'll see why no one would care one way or the other. You won't find a distro that manages to save on RAM by not using an init daemon.
Pretty much every general purpose distro will provide you with the exact same opportunities to run mem heavy or mem lite software and that software will be the exact same on ALL distros. The distros do not write the software. They just configure and package it.
Distros like Arch or Gentoo, which permit for much more customization during install, in a sense make it easier to install minimal systems which do not occupy much disk space. However, it might be observed that since:
The custom install process with those distros is quite a bit more complex and time consuming than it is with distros that use a less flexible installer;
The process of removing components is relatively easy on all of them;
for most people, taking a fat (e.g.) Ubuntu install and paring it down to the point where it is equivalent to a minimal Arch install will probably take less time than just installing a minimal Arch.
This is not to knock Arch or Gentoo. This is just to say that using them "because they are lighter" somehow is a misconception. That's not why they exist and that's not a justification for choosing them over something else.
Further, running the same software stack on Ubuntu or Debian or CentOS or Arch or Whatever will result in the same memory usage.
If you want to minimize RAM usage, there are two scenarios to consider:
I don't want/need a GUI.
I do want/need a GUI.
If the scenario is #1, then you're set. A full fledged linux system without X will fit in ~20-25 MB of RAM, whether it's ubuntu or fedora or arch or whatever.
Of course, if you are running services, those are additional -- but they will be the same addition regardless of distro.
If the scenario is #2, then you want to use a lightweight DE (desktop environment), or a stand-alone WM (window manager). There are configurations with the latter which can run in <100 MB.