Better yet, are there any Spacewalk-like software packages that work for both RedHat-based and Debian-based systems?
As of 2015, Landscape is free for small installations (10 servers + 10 VMs).
Installation instructions can be found here: How do I install Landscape for personal use?
It is also open source, with source code available on launchpad: https://launchpad.net/~landscape/+archive/ubuntu/15.01
You can try out vFense , which is still in Beta, but it supports Ubuntu, Debian, RedHat, OS X and Windows. vFense is an open source cross platform patch management tool, that is also cross referencing vulnerability data from Ubuntu, Windows, and soon RedHat as well. The github link for the project is http://github.com/toppatch/vFense.
Full disclosure, I am the lead developer of the project.