There is no fine grained modifier for control surface faders, particularly with motorized faders that is also conceptually tricky.
An established solution on hardware consoles for this is “Trim” automation mode: record values on top of existing automation with zero dB offset in the vertical middle. But Ardour does not yet support this mode.
If you want to use a physical fader, the only solution I can currently think of would be a gain plugin with a +/-2dB range. Assuming you can control plugins with the “Platform M+”, you could modify “ACE Amplifier” for that purpose [1] (it’s a Lua script, shipped with Ardour).
I’ve not managed to work out how you’d control plugins with the Icon Platform M+ to be honest. Is there anywhere to read up about that sort of thing with control surfaces in general?