Graeme I think your post began as a simple quest to add volume control to your iPod / iPad / iPhone.
I too have an HDX which I control with nServe, and I have achieved volume control indirectly wih Redeye Remote https://thinkflood.com/products/redeye/.
Redeye Remote offer three devices - a simple dongle that you add to your i device (people say it's not that good); a more sophisticated thing that operates as a charger for an iPod (this is what I have); and a still-more-sophisticated whole house system (I don't know much about it).
My amp is a Nait XS. I position the Redeye Remote thing in sight of the amp and that's also where I put my iPod to charge. There is a free Redeye Remote app for i devices and it works by converting IP signals to infra red. It's easy to set up and (a bit like an old Philips Pronto) you can program it to work as a universal remote for a whole range of infra-red-controlled kit. Working alongside nServe to control my music system, the only downside is that you have to fip between two apps to control music selection and volume but it does control volume effectively from wherever I am in the house and it also does things like change the source on the DAC and the Nait XS or, for that matter, turn all the music off and simultaneously turn the TV and Sky box on.
Not 100% perfect but an inexpensive solution if you're otherwise happy with your HDX and the rest of your kit and just want to add volume control.
