Is there a way to hook the amplifier remote power lead(blue wire) on the Empeg to that unit so that the FM modulator is "automagically" on whenever the Empeg is in use?

I don't know how the clarion FM modulator is wired, so I can't answer that question with authority. Sounds like it might work, but don't count on it working 100 percent until you've actually hooked it up and tested it.

Sometimes wiring things that way can induce odd ground loops. If hooking it to the amp remote doesn't work well, you can also try hooking it to the ignition-switched power.

Wait a minute. I just reread your message. You don't have it in your hands yet. I'll bet the Clarion unit has instructions on how to properly wire it. Why don't you just wait until you get the unit, and then read its instructions?

My only question is - is the amp power on signal a constant-on - hot when the empeg is running?

Yes. It is hot whenever the VFD display is on. It is off when the standby light is pulsing or when there is no power to the empeg.