Well I'll be danged, fucos
I had tried what you explained before, but it didn't work.
Well, tried again last night on the way home, and voila, mega hold from the seatbelt.
I was probably not pulling hard enough, or didn't have the seat back far enough to start with.
Man, that works great!
I will probably still keep the CG-Lock though, and make sure I've locked the shoulder portion as well.