Okay, after a little experimenting, it looks like the Press and Hold Down Modifier action while clicking does indeed not work, as you described, but this does:
Click and Hold Mouse with Option.kmactions (1.4 KB)
The real question from here is how best to trigger it in a mouse-only workflow. Ideally, this would work best with a non-Apple mouse with multiple buttons, so you could assign one of those buttons to trigger this macro using the USB Device Key trigger, but I imagine you might be able to use one of the new gesture triggers as well. Otherwise, I'm not currently seeing any good options for triggering this macro without using your other hand to at least press a key to trigger it, though that method would at least let you avoid having to hold down a key for the whole drag like you currently do with Option.