Is there a gate/duck plugin available too that supports a key in like that?

Are you sure you need to side-chain it? I do tons of radio spots every week and I just duck the VO manually using volume envelopes. You don't need to ride it up and down betwen each sentence either. Just drop the music when the VO starts and then bring it up when it's done.

Rob
Hypersound Music & Audio Design
www.hypersound.net

Along with the Native DeX plug, you get a "Native Key Input" plug that acts as the duck input. Then you apply the Native DeX on whatever should be lowered.

A fun sidenote:

On my last game, The Outforce, I used a few tricks to automate boring tasks mostly since I didn't want to do it all over again later with localized (translated) material. One of the issues I had was with the tiny little com-radio clicks which signifies "end transmission". I needed some way to make them automatically, and this is how I solved it:

Track #1: Voices, EQ:ed and ready
Track #2: White noise

I added the Native key input to track 1, and the DeX to track 2. Now I had noise as soon as the voice didn't speak.

I set the threshold of DeX to -inf (remeber that it reacts on silence of the voice track, not the noise) since I only wanted the noise between takes, not just in silence between sentences. (This only works if the voice samples aren't noise gated) Release time on the DeX was set rather high, to allow for some extra silence before the click.

I then added a regular compressor WITHOUT automatic gain correction to the noise track, set it at -96 dB with a moderate attack and long release. This meant that the noise would get cut off right after it'd started, but leave a remaining peak of noise, sounding like a "click".

Finally, I threw in another compressor to shape the resulting "chlich" into a nice, punchy "click!".

Voilá! Automated com clicks! Would have been totally impossible in any other program.