Hi Josh,
You can now POST to this endpoint in JSON, by giving the statement parameter as an array of transaction objects.
This will you allow to create one or more (unexplained) transactions without uploading a CSV file.
I hope this will provide the functionality you were looking for.
Link to the announcement: Improvements to uploading statements
Link to the updated docs: https://dev.freeagent.com/docs/bank_transactions#upload-a-bank-statement
Thanks,
Iain