I’m not sure if there’s sources where you can get a precise marking of which parts are or aren’t included.
The PDF maps here are well… rather vague if you ask me. http://www.viapass.be/downloads/

I see no particular reason to NOT included it, but usually openstreetmap is about ‘mapping what’s there’. Speed limits etc, can all be deduced from what you SEE in the field. Same for weight limitations etc, which are marked with signs. For this particular parameter, I don’t know if there’s actual signage or visual indicators?

EDIT:
your suggested tag seems to work. HGV should be 3.5t and over, which matches our restrictions.