Adding variable __COMPAT_LAYER with value DisableThemes as system variables shows XP-style dialog.
Works only with 32-bit programs.
Shows modern title bar.
Some programs refuse to show dialog. (AFAIK, Notepad++ refused it)
Nice find, really nice find. The default theme overridding the classic theme in the title bar is a shame though. We're on the right track, at least we know it's possible to call change the dialog types now.
Found a similar program to FileEX; XFilesDialog. This one seems to replace the older Windows 95 era with a similar custom one that has more features. Also works with x64 Windows. Not sure if it works with all programs though or just ones tha have that dialog as default. Program is shareware.