If I define my own subsystem configuration in one of the domain controller's profiles,
can the DC/HC push out my subsystem extension module to the managed standalone servers
just like it pushes out deployments to them?
In WildFly's Domain Operating Mode you can define Server Groups in your Domain
Controller configuration . In these Server Groups you say what deployments should be
deployed on which servers, like this:
<server-group name="main" profile="default">
<deployment name="foo.war" runtime-name="foo.war" />
The DC and HC will do their thing to ensure those deployments are installed on the managed
standalone servers. 
Additionally in your Domain Controller configuration, you can define profiles with
different subsystems  (so presumably some managed standalone servers can have some
subsystems, where others in different profiles don't have to have them - or at least
could be configured differently). For example:
<connector name="http" scheme="http"
My question is - does the DC and HC manage that "fictional-example" subsystem
extension module just like they handle deployments? In other words, will that
subsystem's module get pushed out to the managed standalone servers (in the same way
the DC/HC ensures the deployments are pushed out to the managed standalone servers?) If
so, where do you put the module directory? On the DC, like deployments ? If not, how
does this "fictional-example" subsystem that is defined in the profile make its
way to the managed standalone server?