Sure some researchers do pre-register their meta analysis studies on clinicaltrials.gov. It's generally optional. Of course there's nothing stopping a dishonest researcher from doing a little secret p-hacking before pre-registering.

For systematic reviews there are one of two ways that I'm aware of: PROSPERO [1] and Cochrane[2]. Cochrane is slightly more rigorous for conducting a review of this type. Although you do declare what meta-analyses you intend to do, you can't be overly explicit as it is largely dependent on the number of studies that are returned in your search strategy. For example two studies is too small for a meta-analysis so you would do a narrative review (discuss what you found).

[1]https://www.crd.york.ac.uk/prospero/

[2]https://www.cochranelibrary.com

Good practices dictate that preregistration should always be done nowadays, including for meta-analysis. In practice though, this has very little impact. There is absolutely no guarantee that what's written in the paper is a faithful report of what really happened. There's nothing holding back dishonest (or, at that point I guess I should write 'standard') researcher from doing absolutely whatever they want.

how would you know when they actually started the analysis?

That's one of the reasons I was asking. Perhaps they could pre-register prior to being given the full data from other studies?

If the data is kind of public or going around in your circles you could easily hack this pre-registration constraint.