I give you a tip as there is really no need for a plugin
this is the SQL query you need.
SELECT COUNT(*) AS total FROM ow_base_user
One can create dynamic SQL queries from Javascript if not I am sure your get help here as this is very interesting project.
The javascript then can be placed in a custom text/html box or even placed into a floating layer box if you so like.
Let me know how you get on. This should be simple for someone that knows Javascript well.