Update the hub and add Sonos- they're controlling more devices via IP, so I think that what you want is coming. If you use a smart phone or tablet to program these, log into the hub, click on the three horizontal lines at the upper left to change settings- press 'About' and look for 'Versions' to see the software version- should be 4.15.xxx

Why not just replace the Denon amps with a higher model that has 12V trigger?
For the price of your labour trying to locate something that may work, inform the customer that in order to do this properly here is the solution.

You wouldn't have to reprogram their harmony remotes and you don't need to worry about the service calls making it work when it fails.

They are the ones that changed the system after you designed it to work with their original spec. Why can't you change the equipment to make it work with their new spec.