Thanks RAF - I think I understand that, and if I follow your meaning correctly that is not what I am envisaging. I like the tactile feel of a physical controller - either a slider or a turn-knob. When you do that on any current DCC-compliant controller it creates a signal, which signal is then transmitted through two wires, then on to the track and through this to the decoder, and the addressed loco responds accordingly.
What I would love to see is some adapter that tricks the DCC controller into thinking its the track, when in reality it is intercepting the signal, recoding it as an equivalent HM7k-compliant instruction, which is then sent over bluetooth to the HM7k decoder in the loco.
That may be impossible to achieve - or (more likely!!) impossible to decipher my logic - but allow me to dream!