First of all, let's think do we really need to keep icpp-compatibility at all. Or it is possible to forget about them once and for all.

Talking about the solution proposed (to inject registration calls with header - if I understand it correctly), I think this is possible way, but it has one [possible] issue with initialization order. We can't predict the way headers are included and we can't predict the sequence of initialization.

I always wondered why plugins are not initialized inside CONSOLE_APP_MAIN/GUI_APP_MAIN)

with auto-initialization classes