Wix's util:CloseApplication extension doesn't seem to work

Question

I'm trying to close a process before uninstallation using Wix. I've confirmed that it works as long as there's a visible window, but if there isn't a visible window (which is the case most of the time with this app since it's a system tray app), the uninstaller just hangs, and eventually continues with the uninstallation, leaving the process running.

According to this forum post, it seems like Wix has had trouble closing minimized apps in the past, so I wonder if this is related?

Any suggestions as to what else I can do to make sure the process gets shut down? Is there any way I can try and capture the message in my app using the Win32 api maybe?

Here's the CloseApplication declaration:

<util:CloseApplication Id="CloseApp" CloseMessage="yes" Target="App.exe" RebootPrompt="yes" />

And here's the custom action:

<Custom Before="InstallInitialize" Action="WixCloseApplications">REMOVE = "ALL"</Custom>