Ok, so in taking a second longer to think about the problem, in reality the resource manager managing the text for the "Transaction aware Notepad" would need to be a durable resource manager. However in the general case it would still be the only resource manager enlisted in the transactions. This would still prevent escalation of the management of the transaction to msdtc and keep you on the fast track. Although, if you were to do something like "Insert Image" (ok, so we're not talking about notepad any more) it would cause escalation of the transaction to msdtc if the open dialog was transaction aware and the image you selected was on a transactional file system.
Hmm... using copy/paste potentially moves parts of the document beyond the process boundary. Transactional clipboard that supports Promotable Single Phase Enlistment!!!