I made a second call to the FTB practitioner hotline and confirmed that the unused PTE credit is fully refundable on the final trust return.  A Lacerte rep spent over an hour trying to figure this out.  She said that since this credit only gets applied against tax in a normal year, the software won't allow it to flow to page 1 without a tax liability to offset.  Please add a field to the software to allow for this.  In the meantime, I will file on paper and fill out the 3804-CR by hand as well as page 1 of form 541.