I see it as a case that if someone has a need for using a specific library on a Due then, if they have the appropriate knowledge and skills, should be able to make the changes themselves. However saying this, it is also dependant on the actual library and how much of the processor features and registers it uses directly. Anything that uses regular Arduino type calls could be Ok with minimum changes, others using SPI, direct port writes, processor specific registers would be more difficult. other things to consider is that if a particular shield uses SPI then it may be dependant on the SPI pins being on I/O pins 11, 12 and 13.
Other things to consider are changing from using softserial to hardware serial on Due, changing references to PROGMEM and implementing alternatives to EEPROM storage.
For my own personal pleasure I've done a couple, one for Wiznet 820io modules and another currently being worked on for the Roving networks RN-XV WiFi module based on the WiFlyHQ library. I have a number of other libraries that I had created or updated for Arduino and I'd get round to updating these some time but the complexity with these is that they use direct port writes, but nothing I shouldnt be able to fix!
Hope this helps
Andy