I know. That's not what I'm talking about...
No but it does provide what you need.
Look at the memfile implementation. It has a NULL fopen entry because it makes no sense. memfile doesn't open a char* filename, it opens a file handle. So it must include the internal Allegro file header to set the vtable by hand. If there was an al_fopen_f function and related hook in the interface, it wouldn't have to do that.
Sure a helper to play with the vtable may help. But I don't think anything more than that is required. It sounded to me like you were asking for a bunch of new api calls just to support something you can do now.
I don't think thats necessary or useful at all.
memfile doesn't open a char* filename, it opens a file handle.
It actually needs to provide a memory buffer and a length, so it can't just use an al_fopen_f method.