I'm surprised this doesn't already exist, there's no propriety to know if a dialog is on top or not, this is actually needed.

Just to clarify, do you mean if the dialog is on top of all other mIRC windows, or on top of all windows?

As far as I can tell there's no such a thing, either the dialog is on the desktop: it is considered as another application and is not related to mirc's windows, or it isn't and it is always over any mirc's windows.
An on top settings always refer to on top of all windows for me