Hello,
I've found an open source software which use RDM with ftdi devices :
OpenRDM is made for windows, but coded in C++ and it uses the ftdi API so it should be portable.
I'm not sure it could work with OpenDMX dongle from Enttec, but I've tried with my adapter FTDI - USB-RS485-WE-1800-BT, and it works.
I'm not able to modify the ftdidmx plugin to add rdm features, but maybe Rui Barreiros would like to have a look :)
homepage
http://www.hoelscher-hi.de/hendrik/english/openrdm.htmdirect link to source
http://sourceforge.net/projects/open-rdm/RenZO