When you try to make SciTE replace Notepad (through the classic Image File Execution Options->Debugger hack), it receives an extra command-line parameter, namely the path of Notepad itself.
Would it be possible to allow SciTE to accept a /z parameter in the very beginning, telling it to ignore the next parameter? I believe this is what Notepad++ does (or was it Notepad2?) in order to replace Notepad, and it works really well.
I've included a patch; it seems to be pretty trivial, if I haven't missed something.