In the Menu table, have only a global field.
Create a cartesian join (the one marked x) from the global field to any field in the customer table.
Make sure you have a record in the Menu table or this won't work.
Your portal should now be able to search because the join is based on a global record, which is unique to each session for each user, hence will not be locked.
Jul is correct. To explain a bit further, if there are no fields in a table other than global fields, FileMaker never opens the record for editing. If you have even a single field of any non-global type, it will. This is the basis for the Selector-Connector model developed by John Sindelar and Todd Geist.
Thanks for both of your help on this, I've managed to very quickly get this working again using the global field.