Actually, the official ISO EBNF spec (ISO/IEC 14977:1996) can be freely downloaded from the ISO website here. That actually defines meta identifier as following:
meta identifier = letter, {meta identifier character};
meta identifier character = letter | decimal digit;
It refers to 4.14 and 4.15 in this definition. There it says clearly that a meta identifier character consists only of letters and decimal digits.
It is indeed strange that the EBNF they use to specify EBNF does allow spaces in identifiers. I always believed EBNF allows whitespace in identifiers. This is also what Wikipedia says (although the grammar there is incomplete in other ways):
identifier = letter , { letter | digit | " " } ;
So perhaps they have overlooked this in the standard? Or we are both missing something. Anyway, I hope you can maybe figure it out now that you have the official standard.