Nice mod! I'm also 6'5", and when we shopped for RVs we made sure to get one with a residential queen. In fact, when walking through literally hundreds of units at RV shows, DW would immediately cross one off the list if I hit my head anywhere inside. She says that she doesn't want me to accumulate too many concussions because I'm not pretty enough to get away with being dumb.
With that said, our next RV will have a residential king bed. Apparently, my wingspan is sufficient so that I routinely elbow DW in the head while sleeping in the queen.
I know of someone who made a similar modification to a short queen in his RV. He hinged the extension at the bottom and used locking shelf supports to keep it raised. He permanently installed a cushion onto the plywood that matched the mattress in height so that he didn't have to move the mattress down. When night comes, he flips the end up, locks the supports and wrestles with the sheets.