a port of the ultravnc viewer to linux is probably not needed. Vncviewer.exe works perfectly under wine for me, even with the encryption DSM.
I was previously pushing for a linux port of the viewer, because I experienced hangups after a few (random) minutes of activity for vncviewer+DSM under linux wine, which forced me to continuously restart the viewer. I just found that these hangups do not happen anymore when wine is executed as root, e.g. run as:
sudo wine vncviewer.exe
For increased comfort, set sudoers (visudo) to allow that without password

[Moderators: sorry I probably posted it on the wrong forum (even though it can be considered a bug report withdrawal), I don't know how to move or delete this post. You can move it where you think it's more appropriate.]