I did not receive a firm answer on this from Apple support or the forum. I eventually had the time to test this several ways and now have the answer what I believe is the answer. You can publish an iCal calendar using the methods described in the below URL to any read/write enabled directory in your iDisk:
http://docs.info.apple.com/article.html?artnum=61725
Simply change the example publish URL,
http://idisk.mac.com/username/Documents to
http://idisk.mac.com/username/XXXXXX/XXXX/XX
As shown above, this will work for root directories and sub-directories. Perhaps more importantly are the security considerations.
If you publish to a directory that is only accessible by you, with your iDisk account password, only you or a user who has your password will be able to subscribe.
If you publish to the public directory and it is not password protected, anyone who knows the path can subscribe. If you want to do this, it is just as easy to publish straight to .Mac.
If you want to have a number of people have access to a calendar, but not the rest of the world, you can publish to your public folder using the above referenced methodology, with one modification. You password-protect your public folder. The downside to this is your public folder is password-protected for all purposes.
The one that matters to me works well. I publish to the shared folder in a family account. All members of the family can subscribe to the calendar, using their own iDisk user name and password. The rest of the world does not have access to the calendars.
Given the above, iCal is significantly more powerful that I had hoped. If Apple adds one additional feature the potential would be even greater. They need to offer the ability to create folder at the iDisk root, or within the Public folder, that are password protected. This would allow better options for publishing all sorts of data to your iDisk and controlling who gets access, including different calendars.
G5 Tower Mac OS X (10.4.5)