... I am not as concerned about where I mount it as I am on how to mount it. Ideally it would be some way, whether I tour with the trailer or panniers, that would allow reasonably quick and easy access to the bag(s) beneath it, ...
I try to pack my bags in such a way that my handlebar bag is the only bag I have to open during the day when bike touring. I usually strap my rain gear on top of my front panniers, so do not even need to open a bag to access rain gear. The only common exception is a stop at a grocery store necessitates packing the groceries, thus access to the bags.