From a pure user perspective (never looked at the oVirt code for this)...
Right now, it looks like the way to reference the ISO domain is
iso://<filename>. Nothing specifies the domain name (I guess there can
be only one ISO domain).
Maybe an alternative (to get it out being of strictly tied to the one
and only ISO domain) would be something like domain://<domain>/<file>,
so which storage domain could be specified (and then it doesn't matter
if it's an ISO domain or regular storage domain).
Anyway, for my short term, it looks like I just need to build ISOs on
the fly and upload them to the regular storage domain (since I don't
have an ISO domain).