Why can't you?
It doesn't matter if the wire was home run or not. What matters is 'how many' speakers are in the run. You can do up to 16 sets of speakers with some impedance matching volume controls before you would need a 2ohm stable amp. You still haven't indicated how many speakers we are looking at in this system.

The other problem is the 18awg wire. For that, we need to know what the total length of the daisy chain is. And then we can figure how much wattage we could put thru it in a low impedance system.

Again details matter. If this system has 20+ speakers in it and/or total wire runs over 200 feet. Then yes, 70V would be the way to go. Still don't necessarily need stereo VC controls however. Could do just conventional 70V mono.

How much of this is being driven by customer demand?