Why should this be included in the repository?: It's kind of like a retro game creation program. It can be used to play and create games created for Megazeux. It's a very cool, very fun program.
Is it Open Source (yes/no): Yes!
How many users do you anticipate will use this software?: No idea.
Link to source tarball/zip file: http://vault.digitalmzx.net/download.php?latest=src
(The above link downloads the latest source release. As of the date of this package request, that's mzx291src.tar.xz.)
EDIT: Oh, and here's a link to the GitHub page: https://github.com/AliceLR/megazeux