[xmonad] Putting PID into window title?

[Joseph Garvin]

I sometimes spawn multiple instances of app that puts important
information in the titlebar of its main window, but doesn't put the
information in the titlebars of its child windows. When I run two
instances of this app on the same desktop, it's difficult to tell
which child windows are associated with which main window. If the PID
associated with each window were in its titlebar, I could match the
child windows and main windows up easily. I have checked that the app
in question correctly sets the _NET_WM_PID property, but I don't see
anything in Decoration or SimpleDecoration for letting me add my own
text. Is there a way to do this?

Thanks,

-Joe G.