Putting an ATU at the feed point, as we all know, is a great idea.
I have the ATU-10 and I am sure it works the same at the ATU-100 since it's pretty much the same design with the exception of being able to handle more power.
I hope this info helps you.....
The ATU-10 has an interface cable that goes along with the IC705 radio. Since I only need the ATU on the high end of 40 meters, I choose not to use the interface cable. Like your ATU-100, my ATU-10 acts the exact same way. Whether it's next to the radio or at the feed point of the antenna, once it gets RF sent to it and if the SWR is high, it will automatically go into the tune mode. The ATU is pretty much transparent to the operator.
Here is the only down side.....if the ATU can't make a match, it cycles, stops and keeps cycling over and over. It does not just stop. You will be able to detect that by watching the SWR meter in your shack. Your meter won't settle.
As long as you have power to the remote ATU and it's in a weather proof enclosure, you should be good to go. Before finalizing the installation, I would test it on all of the frequencies you plan on operating on to see if you get a match. If you do, you should be good to go.
Half way down my web page I kind of review the ATU-10 if interested?
https://www.ku3x.net/portable-qrp/icom-ic-705Good luck,
Barry, KU3X