straw-man proposal:

• Update the existing Toolforge tool (which already charts coverage for extensions), to write this data to mediawiki.org as a Lua module table (e.g. data points limited to last 30 days). (Or as JSON dataset on Commons?)
• On mediawiki.org, write a Lua module that, given an extension name, can display a singlestat number for the "current" code coverage percentage, with behind it a thin line graph with how it changed in the last month.
• On mediawiki.org, use this in the Extension infobox template.

I would keep tools mostly single-purpose: have the coverage tool expose the data as a JSON API (where API probably just means writing it to a JSON file and making it accessible) and have the extjsonuploader tool deal with pushing it on-wiki along with the other extension data.

When MCR fully lands, we can probably replace Lua with something nicer.

In T220939#5113225, @Tgr wrote:

have the coverage tool expose the data as a JSON API

Filed as T221510: Provide list of repos with coverage information in machine-readable format. For now extjsonuploader just replicates the logic (which is fairly simple).

Change 505432 had a related patch set uploaded (by Gergő Tisza; owner: Gergő Tisza):
[labs/tools/extjsonuploader@master] Fetch coverage data

https://gerrit.wikimedia.org/r/505432