Even with TB you can send messages without a valid address in the To: header, just enter something in the CC: or BCC: header.
The only way to for empty To: headers is filtering the headers for this string:
This looks for a line that starts with To: followed by any number of spaces and ends with a line break, because that's what an empty To: header is, rember to use the 'matches' condition and not 'contains', becayse regular expressions don't work with the latter.
I'm just a user of The Bat! I don't work for Ritlabs.