I personally have found that using tcz's and a tclocal mount does not normally cause problems. The symlinks created by the tcz in /usr/local will persist across reboots, just like the files from the tce. If there are existing files or symlinks in /usr/local the tcz will not overwrite them with new symlinks. But as the tcz is mounted at boot, the leftover, dangling symlinks become live again.
This only applies if you are loading the extensions upon each boot and using the tclocal file/directory. If you are using PPI mode, then obviously tcz's will not work at all in tclocal since they are not being mounted the next boot. Unlike the tce, the tcz extension itself must be loaded and the extension itself must not be deleted for it to work as it's files are only symlinks to the mounted extension.