Just a couple quick notes on this:

• This is something that the community should agree to, since it's on a real wiki. There should be a discussion about it on meta before we do anything, maybe on Meta:Babel. I would suggest starting this discussion ASAP.
• The documentation for this should probably be on meta itself, since it's meta-specific documentation, and not on mediawiki.org (which is for generic technical documentation). In fact, I think the instructions for creating test events could be in Event:Sandbox directly, and other pages like Event Center/Registration could link to it.

Pinging @LWyatt and @RAdimer-WMF for feedback: Hello! We are thinking that one way that people can test the new event registration feature on Meta-wiki is that we create a sandbox event page (Event:Sandbox) and people can create test events as subpages of that page. What do you think of this idea? And, if you think it is a good idea, do you think we need to share this in a forum such as Babel in advance? Thanks!

Update: We have posted on Babel regarding this proposal.

EDIT: Discussed with Ilana - we can go ahead with this!
No feedback yet - how long should we wait before going ahead?

@ldelench_wmf: We have waited over a week for feedback on Babel; I think we can go ahead.