You are correct about the sliding. You can use slide up to close up unused space for a portal row but not for a portal field.
I can throw out a suggestion, but don't really know enough about your report layout to know if it's a workable suggestion or not:
Base your report on your Portal's table and pull the data from your parent record by placing the fields from your related parent record into a sub-summary part that serves as a sub-heading above the data you are attempting to display in your portal. Since you are no longer using a portal, you can now use sliding to slide your multi-line text fields up to reduce the unused "white space" in your report.