I've also played in campaigns where the GM has disallowed OB parrying depending on the nature of the attack and attacker. E.g. a large or superlarge creature might not be parried by anything other than a large weapon, and sometimes not even then. A dragon's bite is not going to be parried by a dagger, and a giant's club is not going to be parried by a mace.
Let's point out, however, that such interpretations are misunderstandings of the RM2 (and probably RMFRP and RMSS) rules, where "parrying" in the rules' terms doesn't mean "to block an attack with your weapon" but merely "(to) sacrifice offensive capabilities in order to increase one's defensive capability" (AL&CL, chapter 4.3) whilst holding your weapon in hand; in fact, the chapter about "parrying"
does point out that "such an action is called
parrying,
though it may be thought of in many combat situations as dodging". AFAIK, only does RMU deviate from such a definition, with parrying, dodging and evading being defined as different combat options.
As such, there's no reason one cannot "parry" in the RM2 (RMFRP/RMSS?) sense a dragon's bite with a dagger, since it merely means focusing on the defense at the expense of your offense ability, such as by dodging rather than attacking with no concern about anything else…