You need to be careful with Resource Hacker though because I'm pretty sure against the TGF2 and MMF2 Standard agreement to alter a compiled program and distribute it. If you have MMF2 Developer, it should be fine but in Standard edition, your extension holds information pointing to Clickteam which you can alter away with Resource Hacker. For this reason, you'd be violating your license by altering your extension.
Your best bet would be to actually contact Jeff over at Clickteam and ask them very specifically, what you plan to do and if this is okay. I don't think he will have a problem with altering the actual ICO the extension displays because neither MMF2 Standard nor Developer have the capability to actually use a 256x256 alpha icon, so you wouldn't be evading buying a feature that's available in Developer since it's not. You'd be compensating for a weakness in the software.