If you have people create records by having the "allow creation of records via this relationship" checkbox selected, then the empty row that people create records in always sits at the bottom of the portal.
However, if you deselect the allow creation of records via this relationship, and give people another way to create the records, then you can have the new record be at the top.
For example, create a script that is along the lines of:
set variable (ID of the customer)
Go to layout (Enquiries)
Create new record
Set field (set the foreign customer ID in the Enquiries table to the ID of the customer)
Go to layout (original layout)
Go to object (name the portal you want to go to and use that name here)
Go to row (first)
Go to field... the field you want them to write into
The trick here is sorting the portal by something that will always mean the empty record is at the top. So you could have a Timestamp created field in the Enquiries table, and have the Enquiries portal sorting by the Timestamp field, in a descending manner, so the newest created enquiry will always appear at the top.
I haven't implemented this with IWP, so I can't say if it all works 100% the same there, but I would imagine it would be ok. Others might have better/different suggestions or perhaps a correction for me.
Thanks for the information - I'll give it a try.