The stated rule is pretty clear: Only the packaged distro client is to be used on Laanx / EZ, period. That said, there are some exceptions that have been permitted from time to time, for example:
1) Player modified UI windows, such as inventory and the like, which do -not- alter the functionality, but merely rearranges the presentation, have generally been allowed. Such mods aren't supported of course, so if a mod doesn't function properly, but the packaged UI does, you're on your own, and if we update the client in such a way that breaks your mod, again you're on your own.
2) For playability or bug-tracing reasons, when we needed a wider range of test cases beyond that which we could do within the team, we may in specific cases suggest certain advance builds be tried by player-testers to help us isolalte and fix the issue. There may be times when we're not prepared for technical reasons to make a full update release to cure a platform specific issue. These are short term exceptions, not a free-for-all waiver of the rule, and certainly once the issue is fixed and published via the updater, the exception no longer applies.
The point is that unless otherwise directed by the team, the packaged distro client is what should be used with our servers. If you want to try to improve the client, build your own server and test it on your own, and submit it to a dev for consideration if it fixes a bug or enhances a feature to benefit the entire player base. You'd be encouraged to discuss your intent with a dev first to avoid wasting your time (and ours), and you probably should consider applying to the team at contributor or prospect level to get a more clear understanding of the development process and direction.