With "close system'' I mean the fact that the paribhāṣās found in the Śrautasūtras are only meant to throw light on further parts of the same text. This is not the case of Mīmāṃsā. As for Grammar, its paribhāṣās seem to be meant for the Grammatical system only (it is only in the Aṣṭādhyāyī that the locative is used to indicate the preceding item), but the way Patañjali justifies some of them, linking them to worldly usages suggests that some of them have a general validity. A further distinction lies in the precise meaning of paribhāṣā as meta-rule in Grammar. As already hinted at (see this post), Mīmāṃsaka nyāyas may be not meta-rules, but just rules.
On paribhāṣās, see also this post and this one (be sure to read the interestng comments as well as the less interesting post).