Import Invoices: missing fields & subtotals

We recently started exploring batch-producing invoices using the Import Invoices feature and an external script to generate the data from our hours log. This mostly works great, except that a few key features are missing, which means we have to manually tweak all invoices after the import. Specifically:

• We have a custom field that appears on our invoice template; however, there is no way to map a data column to this invoice field, so it remains blank on the batch-produced invoices.
• Similarly, while the "memo" data column is mapped to the "message on statement" field, there is no way to map a data column to the "message on invoice" field, so this also remains blank.
• There does not seem to be a way to create subtotals via the imported data, but we need one on all invoices.

Is there a work-around for any of these issues? Thanks!