Now I'm thinking of maybe showing both values one post/author at a time via mouseover and/or after clicking into a post.

Mouseover would be conceivable, but I'm not sure if the data could be loaded (and processed) that quickly.
I can well imagine the information for the post display. The word count and the number of followers could be extracted from the data already available in the Redux Store. We wouldn't need an additional query for this. However, I don't know whether the extension can access the Redux Store. I could imagine that this is not so easily possible.

A lot of the API servers aren't running the follower plugin

The plugin is not necessary for this. This data is also stored in the hivemind database. A (not so simple) SQL query is sufficient for this (The copilot helped with the median function):

SELECT PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY accounts.reputation) AS median_reputation
FROM hive_follows AS follows
JOIN hive_accounts AS accounts ON follows.follower = accounts.id
WHERE follows.following = :aid AND follows.state IN (1,3);

The calculated values were exactly the same as those I had determined with my last test code.