If it were done via house placed chest the log would record it all. That should help tack down any offenders. Second does muleing apply?
Correct. Housing chests have a log. You also specified something placed in Center (or some other common/public area), which would not necessarily be a persistent container, so I was being inclusive in my answers and inquiries.
In order for anyone to take the current item they must place a higher (lens) value item in its place. Isn't muleing giving something to another character (of the same player) for nothing? Or at an unfair price?
...based on the honor system alone. Unless we code something specific for it, there'd be nothing enforcing this requirement. We can talk privately about how muling could be achieved, but I won't go into it here. Trust me when I say it would be really easy.
Using a housing placeable in a publicly-accessible section of a guild hall would mitigate most of the potential problems and be persistent across reboots.
My curiosity still remains though on the question of what happens when an item can no longer be upgraded (or no one wishes to trade out)? What is done with that item?