The HWND for this window is returned by InstallOptions. You need to pop it right after MUI_INSTALLOPTIONS_INITDIALOG.
As for the branding text, I have a new idea. The only problem is it will change the color of the license box too... It's the SetSysColors function, you need to call it with System.dll. I don't have the time right now to write the specific code so you'll have to try it on your own.