Ed,
Look at what the new Boxster's have in place of the spare tire, a 12v compressor and a plug kit. That's what I would suggest. You can pick up a plug kit anywhere and ether get a 12v compressor or a couple cans of Fix-A-Flat for air.
This is a link if you don't know how to plug a tire;
http://www.alpharubicon.com/bovstuff/tirepluguzi.htm
For peace of mind, check your tire's to make sure their in good enough shape to make the trip. Then give them a look over every time you stop for gas, bathroom or just to stretch your legs at those scenic overlooks. If the cars loaded, double check the tire pressure. You might also think about the time of day and time of week you travel. If you do have a flat that can't be repaired by a local (removed and patched) replacement might be another story. I have a S and finding the tire size I need (225/40-18 & 265/35-18) in the middle of nowhere would be a nightmare.
Like the past post said, if you did put on the spare tire where would you put the flat? I think if you follow the rule of 6 P's you should be fine. The 6 P's by the way is " Proper planning prevents piss poor performance". If you plan your route, have phone numbers of contacts along the way, have a plug kit and inspect your tires you will have fulfilled the 6 P's.
Have fun, wish I was going with you.