How to integrate previously proprietary data into OSM?

I am redeveloping a website that displays local churches in an area. Previously they hosted their data in a private database and used Google Maps to display it, but as an avid OSM contributor, I considered to merge their database into OSM and then use OSM data for the website to foster proper collaboration.

However, they track and allow to filter by some extra information, such as the rough amount of visitors each week, whether there is a kids service and in which language and style the service is conducted. I would like to add these under custom OSM tags to enable others to benefit from and edit it, but I am worried that some contributors might remove these as they are not officially documented, thus degrading the functionality of the developed website.
Of course I could also track only this extra information in a private database, but that kind of defeats the point.

What would be a sensible way forward? Note that a months-long proposal process is not an option, and it doesn’t matter whether these tags are made official - just that they stay on. I would of course try to follow a sensible scheme so that they are mostly self-explanatory.