Just for fun I threw together a repeater program in c# that works on Windows 7/8. It also sets the RTS/DTR (or both) pins high when transmitting the audio.
I wanted to make the repeater only transmit if a certain nac is decoded rather than setting audio levels. Since i'm not as smart as the people working on DSD+ or OP25, I decided to go another direction.
After some reading, I managed to grab the text output from DSD+ while it's decoding, search through each line as it comes in, and transmit based on the NAC it decodes
It's working very well! I'll post a youtube video tomorrow.
The other idea I had was to expand on the repeater and use it with NXDN / etc...
As long as there is some unique value output by DSD+, you should be able to use it as squelch.
I tested my repeater with a discriminator tapped pro-95 and the TX audio goes to the 6 pin mini din port of my yaesu ft-7800r.
Just thought i'd post for comments / suggestions. It's a work in progress.