I have learned that the neuter nominative and accusative hoc is actually pronounced as if it were hocc. But was it exclusively hocc? Was it ever pronounced as the hoc that it looks like in the classical era?
Based on metric evidence, it was always pronounced "hocc" with a geminated c, but still spelled hoc.
I went through all the 124 appearances of the hoc in Virgil's works. Whenever hoc was followed by a vowel, the scansion unambiguously required that the syllable be long. Many of the 124 instances occurred before consonants and there were cases of the ablative hōc, but there was still a substantial amount of cases of the nominative or accusative before a vowel.
I also checked about half of the 552 appearances of hoc in Ovid's works, and the conclusion was identical. There were plenty of cases where the length (weight) of the sole syllable of hoc was visible before a vowel, and in every single case the syllable had to be scanned long. Therefore I feel I can confidently conclude that one always pronounced hoc with a geminated (long) c in classical Latin.
This gemination concerns pronunciation of hoc before a vowel. Metric evidence will tell nothing about the length of the c when followed by another consonant. It is possible that the c was short in such cases, or that the length was not such a meaningful concept in that context. The pronoun is scanned long anyway in poetry.