No answers yet? I have been watching to see if anyone else would chime in as this sounds like a great idea.
I was curious as to how the DOS DDR senses how full the reservoir is. After watching the video that is on the Neptune site, I see that it is just mathematically figured which could only be done using a dosing pump with a measurable accurate output (such as DOS). I.E., you manually enter the starting level of the reservoir in Apex Fusion. Then as the DOS unit pumps out a given amount of fluid, it subtracts that from the initial volume. This only simulates an actual fluid level sensor. The DDR relies on the fluid optical sensor towards the bottom to send an alert when it gets low.
So I'm in the same boat as you with an ATO reservoir that needs filling every so many days. I have a optical fluid sensor towards the bottom that alerts me when it's time to refill. The thing is, I don't use any kind of dosing pump for the ATO, just the Apex PMUP pump which is effected by gravitational forces. Therefore I can not accurately measure how much is being pumped out, thus I can't subtract it from my initial water level. Plus I believe, from what I saw on the video, that 2 liters is the max that Fusion can be set for, although I could be wrong on this. So even if you were to use a DOS pump you may be limited to that initial volume. (Again, I'm not 100% sure of this.)
What would be possible would be to put a number of sensors at different levels in the ATO reservoir (like high, medium and low), but now I'm just cluttering up my Fusion screen.

Perhaps there is another way to do this so I'll just keep watching to see if anyone else chimes in. ;Watching