I am trying to do the following:
copy-jre
You can use profiles to do this.
e.g.
<profile>
<id>platform-windows</id>
<activation>
<os>
<family>windows</family>
</os>
</activation>
<build>
<plugins>
...
</plugins>
</build>
</profile>
In your case, you might just want to specify os/family in the profile's activation element.
The Sonatype book, "Maven: The Complete Reference" section 9.2 deals with maven properties and lists the following properties that are pertinent to your question:
java.version
os.name
The java.version property seems usable and in my testing returned "1.6.0_27". However, the os.name property is not exactly an "os family" as you requested. In my case os.name returns "Windows Vista", which I imagine is not what you are hoping for. I don't know of a property to get the os family as you desire it so I recommend using maven profiles as prunge mentioned to handle configuring your plugins with the desired os.