For what it's worth, an update on this thread.
When six months passed without a word from Honda, I wrote
If Head Office were really looking into this they would have been in touch by now to ask, for example, for the bike's serial number or to see the actual faulty part. Instead, it seems the problem is just being completely ignored.
They replied (my formatting)
Please rest assured that our internal teams are looking into this. However, we cannot include clients in these processes. The information and photos that you have provided us are under investigation, as well as the tools. equipment and materials used to manufacture them.
Please accept our apologies if you feel left out from this process.
to which I said
You will understand that I need to see evidence of the investigation. It's not good enough for Honda to say they are looking in to a problem and not feed back to the person who showed them it.
I had to go to a bike breaker for a replacement part but, as I have said before, I am not seeking any compensation. I'm only interested in the safety of motorcyclists and just want an explanation how this part ended up on a bike sold in a Honda showroom.
The customer service chap said
I will pass your request up the line once again.
and I think that's the last I will hear from them.
Make of it what you will. I was going to make an angry YouTube video but I really can't be bothered. Sometimes you just have to admit defeat. My last thought is maybe I could send the part to a motorcycle magazine and see if they are interested in it?