Yes, I recognise this feeling. I tried writing in advance, but discovered that the incompetence of the hospital meant I had wasted my time - the letter had not been read by the specialist despite it having arrived two weeks in advance. I also found that several specialists we saw lacked the skills to manage the complexity of collecting information both from the individual with dementia and their carer...the ones I saw seemed to be bogged down in considering the rights of person with dementia to the point of trying to force you into an openly confrontational position when something my Mum had said was simply untrue rather than use exploratory considered questioning or reflection to elicit a rounded picture in a helpful way. The Trust providing the MH services in my Mum's area was, in my view, very very poor.
If I was in this position again I would a) insist on a short separate discussion (perhaps while someone else did the usual basic health checks for my Mum; and/or b) write and send by recorded delivery. None of this is easy to do when you are coming to terms with the impact of dementia on someone close to you, which is why I didn't do it at the time. I didn't have the energy then, nor did I think that being "confrontational" would help my Mum. Long term, though, this has just let me looking back and thinking about how much better things would have been if I had.