Depending on wordlist and whether you want a 16 or 17 letter word, you get "charitablenesses", "supernaturalised" (British spelling), "quadricentennials" or "quartermistresses". These boards all score considerably lower than the REPLASTERING board. Full results here: https://github.com/danvk/hybrid-boggle/#highest-scoring-boar...
I hadn't realized until I did this "side quest" that most wordlists top out at 15 letter words. That makes sense for a Scrabble dictionary, but it's not great for Boggle.