This depends on what the exact safe mode settings are on your server:
Have a look at the different safemode PHP settings: http://lu.php.net/features.safe-mode
Some examples:
safe_mode_exec_dir can prohibit access to the imagemagick executable.
open_basedir can prohibit PHP to open a file in the tmp folder disable_functions and disable_classes can disable core PHP functions and classes that EZ needs to work
All these settings can be set by your provider to his liking so it is impossible to say whether EZ will work or not. I would say that you can expect to have no access to imagemagick, which means you need gd. Access to all directories within the ezpublish tree should not be a problem. Access to a tmp directory might be a problem, but any halfway decent webhoster will give you such access as otherwise you cannot upload any files using a form.
I am not sure about the effects of the Safe Mode GID setting. There might be problems if you upload files using ftp (e.g the initial upload) making the ftpuser the owner, whereupon PHP can not open the files, but this depends again on the exact setup of your server.
Just to let you know, I have done some tests with your suggestion and managed to succesfully get eZ to work with safe_mode on an enviroment very close to the one I mentioned except on Linux instead of Solaris.
I will keep on trying to find areas of the system that cease to work with these restrictions.
If anyone is aware of any other pitfalls that may be caused by using safe_mode, I'd appreciate to let me know.
Regards,
Felipe
Felipe Jaramillo
eZ Certified Extension Developer http://www.aplyca.com | Bogotá, Colombia