500 khz is only remembered by a dwindling group of former Marine Radio folk which will be non-existent in about three decades max, so, realistically (not like me!), what would be the point of making a memorial of a silent frequency?
Getting into range of that 500 kcs cacaphony in the Channel was a good sign that you were nearly home, but ... what a racket!
There is already a generation of seafarers who have never sailed with an R/O, maybe I could get myself designated a "heritage site".
PS My theory is fading fast, but something tells me that 500 kcs wouldn't be much use for mobile phones.