This is true, the portal object as a whole can slide, but not objects within it.
I think that you are describing the oft recommended option of creating a layout for printing that is based on the portal's table with fields from the parent table included in the header, footer etc as needed. You use a find or go to related records to pull up a found set of records on this layout--set to display the records in list view with the fields that are in your portal row place in the body of this list layout. Since they are no longer within a portal, they will slide.
The invoices starter solution that comes with your copy of FileMaker uses this option to print invoices so you can dissect that solution file to see the details of how you might implement this option.
Thank you Phil!
And especially thank you for pointing me to the invoices solution that I can look at for an example. I'll give this a try in the morning.
Also - for anyone finding this post with a similar issue - I kept googling after I posted my question and found another option. I went to the related table that I was showing in the portal and created a new calculation field. That field concatenates all the fields I wanted to slide left so they would display together. Then in the portal field in the print layout, I chose this one field from the related table instead of all the individual fields. Looks like this works reasonably well. But I'm going to try the other option too and see how the results compare.
One of the key advantages to printing from the list view layout is that you can print out a report that displays any number of portal records without any possibility that you failed to make a sliding portal tall enough to handle even the largest number of related records.