I know you asked about big tires with no lift, but I am going to answer a different way.
Check out the attached photo of my DS with a Club Car 3" lift, and 23 inch tires. It seems to me its got alot of clearance, so you might be able to fit 20" tires with no lift.
There are ways to do a cheap lift on a DS, you can add a block on the front end and lengthen the shackles on the rear.
Here is 1 example:
http://www.ebay.com/itm/CLUB-CAR-DS-...4AAOSwAYtWF8Kb
Aflyer