Hi Jo,
This is a great question and a great design pattern. We do what you are doing a lot with SharePoint forms to reduce locking contention since most people just want to add comments. We also have examples of doing this with SQL using our Database Accelerator.
Out-of-the-box, InfoPath only supports submitting to one database via it's main data source. You can use secondary data sources to do the submit but that will require coding the SQL connections yourself. The SDK has some examples here, but expect to spend 10-20 hours of dev time getting it perfected.
If you don't want to do any coding, you can use our Database Accelerator (evaluation version available at Downloads->Free->Database Accelerator). This will support multiple databases and tables.
Cheers!
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra