Hi Keith,
Not sure if this will help but a couple of years ago I migrated from an SQL_ASCII encoding to UTF8. The data was primarily ASCII with some windows garbage, and a little bit of UTF8 from customers filling out forms that were not specifically encoded anything.
I wrote a utility that in-place scans and updates the tables in your SQL_ASCII-encoded database and ensures that everything is 100% UTF8 NFC at the end. For us, there were some characters in some bizarre local encodings, and we had to either toss or make educated guesses for them.
After the cleaning, you dump with client encoding UTF8, then restore into the final database with UTF8 encoding.