How to syspatch from a different server?

[Federico Giannici]

We use an internal server to speedup the upgrade of OS and packages of 
all our internal machines (all amd64). We simply set /etc/installurl in 
every machine to point to our server were there are the OS tarballs and 
packages.

Anyway we'd like to use the standard OpenBSD servers for syspatch as it 
uses much less files than complete installation and it's often updated.

How can we do it?
Is there a way to tell syspatch to use another source instead of the one 
in /etc/installurl? If no, what about a new option in syspatch?

Thanks.