Thanks for the link, but it only refers to what to do once you have your plugin svn directory already allocated. I have tried changing the plugin directory by changing the BP_PLUGIN_PATH, but that only breaks the plugin as anybody doing an automatic install will have there plugin files in the same svn directory. I have done everything that can be done other than submit a totally new plugin with a different name so it gets assigned a new directory. Not something I really want to do.
The author of the other plugin is better placed than me to change their install directory as he has complete control, since he is working outside the wordpress plugin system. I am surprised after all this time he has not contacted me.