Hello, Sam. (?)
Yes, you can do this. Here's how:
1) Create a global field in your target (i.e. related) table to hold the user's selection.
2) Create a calculation field (for example, "portalSort") in the target table. Make it look something like this:
Case ( globalField = "yes" ; FirstName & " " & LastName ; globalField = "no" ; LastName & " " & FirstName )
3) In your portal dialog, tell FileMaker to sort the portal based on "portalSort".
Caveat: Since "portalSort" will be an unstored calculation, this method may be somewhat slow to sort, especially over a network. So use with care.