What tools would one use to make a custom Live DVD out of the existing Mageia Live DVD and how would one go about it? What kind of guides are there? Are they specific to Mageia or would the same work for another community distro like Fedora?

I would suggest you do an online search for 'remaster mageia'. If you want to remaster an installed system with just the software you want or have added, Debian and Ubuntu have versions of remastersys. The Debian version works on Debian, the Ubuntu version on Ubuntu and many of its derivatives. PCLinux has probably the simples version which works only on an installed PCLinux. PCLinux was originally based on Mandriva so it would be similar but I would not really expect it to work on Mageia.

If you want to modify the Live CD without installing, that would be a different matter.

It's the latter. I want to make a modified Live CD with customized splash screen, desktop background and +/- a few apps.

The link below to the Mageia forums doesn't give one much hope. There is a link in the second post but get a forbidden error, might need to register at the forums to access it:

https://forums.mageia.org/en/viewtopic.php?f=8&t=1444

Or it's a broken link? Hi. You can use tools like Arc or 7zip to look into ISOs but need other tools I'm not familiar with from there, good place to start would be: http://www.linuxfromscratch.org/lfs/, http://lmgtfy.com/?q=make+your+own+linux+iso, http://ubuntuforums.org/showthread.php?t=1528562
and yancek's advise seaching for remastering Linux ISO... GOD Luck.

This link is not broken. The name is up for sale. The site "mandrivauser" no longer exists.

I believe all install images are ISO9660 compliant.
http://en.wikipedia.org/wiki/ISO_9660

Therefore the basic outline of how to modify a Debian image should give some idea of how to proceed.
https://wiki.debian.org/DebianInstaller/Modify/CD

As was stated before there is, or was last time I had a look at it, a remastering application for PCLOS. This might work but they use a hybrid package management system using .rpm packages but using their own script to use apt-get and synaptic (Debian tools for .deb packages). It might be worth taking a look at but I doubt it would work without some serious modification to the script(s).

I know Mandriva had such a thing around 2010 but can't seem to find any indication that it exists now. You could check their documentation. I think this is gone though. I think if it existed I would be able to find some mention of it on the Mageia forums. Can't do it. Could be searching incorrectly.

I know you can extract an ISO and edit it, rebuild it to the standard (ISO9660) and, with luck, have it boot correctly. Am not on my Mageia drive currently, haven't been for some time, too busy, so I can't check to see what packages are available for converting a file system to that standard. Should be something there or in the Mandriva repo though. They have to have one to build their own standard ISOs that you are trying to modify.