I would suggest that using ftp logs stored on the client machine is not a particularly good audit system (not for security anyway). If you want reliable audit logs from a security viewpoint, I would start with not letting people being able to access and modify their own logs.

Having said that the solaris ssh server, and the openssh server for solaris dont have support for detailed logging anyway. There is a home made patch or something for the openssh one, but I wouldnt put that on a critical system (the kind that you would be interested in logging in the first place).

Perhaps using a WebDAV server with SSL would be better. There are clients for all those O/S's you listed plus the server will log the information you wanted.