I think you already have your solution! Use the BizFormItemEvents.Insert.Before, and from there you should be able to grab your
e.Item.GetValue("MySVGField") and processes it, then simply set it to empty afterwards (
If you are looking to be able to send/processes this without even having the database table, that may get a little trickier. Fields without Database representation don't make it to the event logs, you would need to potentially us a Field Without Database representation and use a custom Online Form Control so you could Validate the form, get the SVG value from the control, then submit the form, and processes your SVG after in the webpart code.