Oh, and how do they react to methylation?
I doubt that they would form any codons at all, since the tRNAs bind to specific codons. They could bind tRNAs, I suppose, if they had the same H bond patterns on their edges as natural DNA bases. But then, why would one need non-naturally occurring bases?
If you wanted to create a whole genetic system—the base pairs, tRNAs that recognize the triplets, enzymes to charge the tRNAs with novel amino acids—I suppose that is possible as an academic exercise. Not sure how useful it would be.