Are there any methods that I can use to set the "subject" field
encoding?

In the MMEncoder:
if(m_Message.isSubjectAvailable()) {
m_Out.write(FN_SUBJECT + 0x80);
m_Out.write(m_Message.getSubject().getBytes());
m_Out.write(0x00);
}

is the + 0x80 related to the encoding?