ooh, think i've sorted out the main problem. i was hitting the 512 limit on textures, but not with used ones!!
visitor does not automatically purge unused textures, so if you texture your island nicely using many textures and transitions, then paint it all one texture, all the transitions will still be saved into the pew and wrp file!! my island only used 433 textures, but was trying to save ~900 from previous layouts. as the wrp can only hold 512, some of the required texture names don't get saved into the wrp and hence appear as a hole in the ground. Visitors texture manager can purged these unused textures + transitions. cool : )