This might actually be a safety feature that you have set.
When I try to set the NSWindowLevel of the webview to
-2147483622 the BTT interface replace that value by -2147483648
This is for version 3.298
Thanks for those amazing feature,
PS: according to http://www.gietal.net/blog/nswindowlevel-shenanigans this is the kind of value that should allow to put the html below the desktop icon (which is what I try to test)