[ElMiko]

The .dic file is overly permissive. It includes extremely obscure or archaic words (like "bouse") that are going to result in false negatives for common misspellings/transcription errors (e.g. "house"). As you said, to add words to the dictionary, the custom word list is a perfectly adequate solution. But I'm not trying to add words; I'm trying to remove them from the dictionary's whitelist.

Thanks for the tip on "affix compression"!