Never done it but I don't see why you couldn't. The standard is 3/16 braided so you could step it up to 1/4". Bigger than that you might get a little mushy and have a hard time feathering when the rope is under tension. You may be able to stretch the hell out of 5/16 and not get too much mush using cobras. Bigger than that and I would think you'd get mush for sure.
You can also "tune" your cobras to work with your existing diameter.
If you don't already have cobras, the other thing you could do is order some of jamesthomas gimbal locks for the same price as cobras and then your smaller diameter oars would be fine. They are smooth as silk and give you range of motion the cobras only wish they could.