As of today, the drop-down list for fields to search only contains Title, Author, Subject, Barcode, and Call Number.
The only way that I have found to access a publisher's agent page is through the resource page of one of their books--a great inconvenience when attempting to remove duplicate agent entries from the system.
The ability to also search by publisher would be a practical and useful addition to the user interface. I hope it's a fairly low-hanging fruit that can be implemented soon.