I'm unsure if
this is related but I'm getting similar problem.
I use a french azerty keyboard configuration, to generate some accentued letters, I must input a key for the accent first (for example the '^' key, and then the letter. When pressing the '^' key, on keydown triggers with a $keyval value of 24158 (the character is $chr(24158)) whereas when subclassing the window, no keydown event can be observed.
I'm using Windows 7 and mIRC 7.32, you can easily reproduce that by settings a french keyboard configuration (control panel > regions and languages > keyboard and languages > change keyboards.. select french and 'add', then use the left shift + alt shortcut to get that keyboard and hit the '[' key on a qwerty keyboard).
