I don’t have an LD-5 to do any testing on, but I think I understand what’s going on here so I’ll at least make a stab at it. Something to go investigate is better than no leads at all, right?
For reasons that are not clear, it seems the OS X Prolific driver likes to set the DTR line low at initialization. For lots of target devices this doesn’t matter, but on the LD-5 (and I’m guessing here) it seems the DTR line is used for the PTT signal. This is actually pretty common for rig interfaces.
The experiment I’d do: download a serial control app such as ZTerm or Serial, and use the app to open the serial port and force the DTR line high. At that point, the LD-5 should register that PTT was released and receive as normal.
Links to the apps and other OS X serial port troubleshooting info here:
http://pbxbook.com/other/mac-ser.html
You may also have luck using “cocoaPTT”, a tool to integrate with AppleScript for PTT via serial port. I think you can configure it to release the DTR line automatically, but I haven’t played with it. http://www.w7ay.net/site/Applications/cocoaPTT/index.html