Hey Dan, thanks for looking more into it!
According to the wiki, it really feels like a time capsule does not fit in the description of a memorial: "A feature for tagging smaller memorials, usually remembering special persons, peoples lost their lives in the wars, past events or missing places."
It's generally accepted in OSM to create new, informal tags, and that's often how they start getting used and accepted before they are officially endorsed by a vote: https://wiki.openstreetmap.org/wiki/Any_tags_you_like

I would actually be more in favour of creating a new one, as making use of one that is rendered _because_ it will show on this map can be considered "tagging for the renderer": https://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer

Using a dedicated tag that is (somewhat) already in use, and that makes semantic sense, like "historic=time_capsule", is a good idea because it will allow data consumers to find these things (instead of having them mixed with other unrelated features).

Would you agree with that?

And well done finding the artist :)