While you may have trouble finding a document that says you do not require to be wearing a killcord at all times, the other party will have trouble finding a document that says you DO!
If I were in your position I would take the following steps:
1. Ascertain if local commercial operators of planing hardboats with full cabins use killcords. If not, I'd ask the person who is demanding that you do: "What makes me different?"
2. On a diesel cabin RIB - I would produce the Engine Manufacturer's Manual that (almost certainly) will give dire warning about the effects of stopping a diesel engine via a "power off". I would point out that killcords often get pulled accidentally and a big diesel could take time to get up and running again.
3. I would approach the manufacturer/s of such craft and ask them about the number of killswitch devices fitted to their products recently (very few to none is the answer) and I would include this data in any argument I was making.
4. I would fit an Emergency Stop button to my panel (or refer to existing) to cover situations where a crash stop MIGHT be required - maybe an MOB in a bow-on landing situation or the skipper suddenly becoming unwell.
5. Lastly, if all else failed, I would make wearing one part of my Operating Procedure but only while the boat was on the plane. I would fight it's use in displacement mode tooth and nail, as it is counter productive and IMO, increases the chance of a problem occurring.
6. All the above notwithstanding, if it was in Ireland, we would simply fit a killswitch and then not use it when no-one was looking