As you doubtless know, many EU countries don’t require the tractor reg plate to be duplicated on the trailer. Any speed camera taking a photo of the rear of the vehicle won’t show which tractor it is, at all.
So it must be the norm in those countries for the fine to be sent to the reg owner of the trailer directly?. I doubt there is any wriggle room for you.
You may have stood a good chance in the UK saying it wasn’t you under a particular trailer if your tractor plate wasn’t showing, but probably not there.