You're still looking at overall diameter as the main limitation, and I'd think 32" would be the max. Toyo (Open Country), Nitto (Grappler), and Cooper (Discoverer...AT, I think?) make aggressive tires in a 285/60-18 that might work. Here's a link to a great tire size calculator. You can play with the numbers on any size tire and wheel...
http://www.miata.net/garage/tirecalc.html
Good luck!
Take care,
Jerry