[exim] SQL hostlist against a different server

Hi all,

I've noticed since 4.68 , I can use different SQL servers for
read/write, or define different servers on a per-query basis, as
coveredin the manual in 9.21/9.22 [1]

However.. I can't see how I'd query a certain server for a hostlist.
For example, how would the below be modified?

hostlist blacklist_hosts     = mysql;Q_BLACKLIST_HOSTS

Thanks!

[1] http://www.exim.org/exim-html-current/doc/html/spec_html/ch09.html#SECTsql
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