I am using a header template and would like to display information within that header that is contained in custom properties in the windows where the header is used. Is there a way to get to this information with an expression? Seems like I must be missing something simple here.
You should try using a bidirectional binding to a Client tag. Whatever changes that are made in the header window will be propagated to the Client tag then you can read from that in your other windows.
If you prefer, you could use system.gui.openWindow to make a connection to the header window and then use getComponent to grab the data from the other window:
winHeader = system.gui.openWindow("Header")
myComponent = winHeader.rootContainer.getComponent("myComponent")
#forgot to add code to get the custom property, here it is:
myCustomProp = winHeader.rootContainer.myCustomProp