I didnt understand what you want to do exactly Warren, you want your code to execute before the original application code? If yes, you can just add your assembly code to end of pe and do a jump to and back. Also i didnt understand what you mean by illegal operation, if you are just trying to intercept erros in a debug application then you could just use dbghelp to dump stack details and save a crash dump report to later analyses and replication of the process. Please explain and i will give you more details.

Cya