I think they are indeed coordinated to a degree, altho how that happens I don't know. I do know that every station is given a certain time limit on each frequency to complete their transmissions, so that does minimize the possibility of QRM.
By the way we have a number of other sites with NAVTEX and SITOR-B (the mode which NAVTEX is based) information here...
BTW, that's a nice page you put together...it partially inspired me to finally get going on my NAVTEX setup, which is something I've been wanting to do for a long time.