I think the title makes it pretty clear but:

I want to register a block etc after the Minecraft client has fully loaded (ie. after startup; pre-init; init; post-init etc ...)

Thanks ^^

When asking a question you should explain why you're asking. There is probably a proper way to achieve what you want. For example, if you just want to enable a block in the game based on user settings, that is quite possible, but not by mucking with conditional registration. Let us know what you're trying to do and we can advise on best way to achieve it.