Sorry, but this is not possible in FileMaker.
There are, however, ways to print your invoice that do not require the portal you used to enter the line items on your invoice layout. You switch to a list view layout based on the line Items table and add fields from the Invoice table to the layout's header. Since this layout does not use a portal, you can set the fields in the layout body to be several rows of text tall and to "Slide Up, Resize enclosing part"--options found in the position tab of the Inspector in FileMaker 11.
Here's a simple invoicing demo file created by Comment that uses such a method:
The Invoices starter solution that comes with Filemaker 11 also uses this method.