Whether faxed documents are subject to E-Sign is a debate unto itself, but, assuming for the moment that they are, is an extension agreement or modification a "record" under E-Sign? It's not a disclosure required by regulation...but is it a document in writing required by law? Thanks in advance for any input!

An extension agreement is a contract document. It is not a required federal disclosure. In order to establish evidence of agreements, contracts are usually memorialized in writing. Assuming you want to create a document for this purpose and transmit it to the borrower, you are free to rely on your state's UETA as the legal authority for what constitutes a valid "written" document. Only when you are dealing with federally-required documents do you need to go the ESIGN route.