These products, although no longer under development, as you mentioned, are still being manufactured. If this is the case, then there exist documentation in order to create the product. I do not think it will be too difficult to use these documentation in order to create a DHF. At the end of the day, a DHF is a document that entails a list of requirements needed in order to build the device. Therefore, it is not difficult to figure out how the device is made if they are still being manufactured. They are just not developed and no changes are being made to them. Also, if no documentation exist on how to build the device (which is unlikely), reverse engineering the device would allow us to gain enough knowledge to write a DHF.