Last ditch
Because I don't believe in the acid argument and modern enamel, I don't bother with it. I cook lots of acid foods in enamel without the slightest difficulty. When confronted by really dirty (but not chipped) flea-market enameled goods, I do the oven cleaner trick, as described above. If it's beyond that, I put it in the GE P7 for a regular cycle.
I, know, I know. But - we're talking last ditch. I've used one such piece for a few years now and none of the horror stories about micro-crazing and cracks and chips have come to pass. And that, after many many tomato sauces and lemon reductions.
I wouldn't do it if there were any other hope, but, boy does it work.