you could instead simply create "assets" directory on your local network (somewhere) and then use "Import" > "Advanced" linking feature in Flash Builder to add linked assets folder to each of your project. it *will* be included automatically in application build preferences and within "Package Contents" (available later under app directory). This is common way to share assets accross different Eclipse projects (I believe it will work as drag&drop feature on Windows by dragging folder into open project in Eclipse - but on OS X it simply does not work that way as directory is simply copied into new project instead of asking for linkage option - so File>Import menu should be used).
This is long, established feature of Eclipse IDE, see e.g.:
It's actually a lot easier than you think.
Just reference the file like any'ol URL using a path relative to the SWC's src directory.
So if you include the file "assets/xml/some.xml", just use that same string like you would any remote resource.
var loader:URLLoader = new URLLoader( new URLRequest("assets/xml/some.xml"));
I believe it would also work like this "/assets/xml/some.xml", but I prefer relative paths so the link doesnt break if moved out of the SWC...