Code Injection

NP ;-)

A good Solution would be coding an own Loader that breaks when the Exe is unpacked and goes then to inject your Changes...
R3 Functions like - CreateProcess,Set/GetThreadContext,Write/ReadProcessMemory will be enough for this.

This Way you can make sure you patch it right after the Data is unpacked, at some secure Point you catch and hold in the Unpack-Routine.

I have put some Unpackers with Sources on my Page,maybe they help you getting up a bit into it.

xxx.cstn.cjb.net

Cheers