Hi,
Unfortunately this is not easy, there is no way of modifying the selected parameter other than via the numeric keys.
The only way would be to reverse engineer all the sysex commands for every parameter of every effect and create a full blown editor, this would be a major piece of work.
It is possible in the future that a second screen could be added where you could have slider or knobs to control any parameter mapped to the general external controllers 1-8.
Cheers
Andy
