Currently, it is possible to search the global rename log via Special:Log with a [[ https://meta.wikimedia.org/w/index.php?title=Special%3ALog&type=gblrename&oldname=CardiffCove | query such as this ]]. That query (as of the 2018-05-02) returns a single result.
It is also currently possible to list the log events in the global rename log via the API, using [[ https://meta.wikimedia.org/wiki/Special:ApiSandbox#action=query&list=logevents&letype=gblrename | a query such as this ]]. However, it is not possible to filter the log by the old username via the API.
As an API user, I would like to be able to search the global rename log by old username using the API.