I would try scoring the area where you will cut, outside the cut area so hopefully the cutting process won't chip the enamel. Make sure to support the top to keep vibration minimal. Perhaps you could practice on an enameled bowl to see if it works.

aliceinwonderland,
Thanks for that advice. I hadn't thought of scoring! But... just how would you do that?? I had thought of taping the area off?? I've seen some old enameled tops by themselves at garage sales... should've thought to pick one up to practice on!! What type of blade do you think would work best?

Hate to be a pessimist, but...

To score the enamel, I'd use a circular glass cutter. Talk to a stained glass supply shop.

The problem I see is that if the metal is thin, it will flex and vibrate when you try to cut it, and shatter the enamel, and if its heavy, you are going to need something like an acetylene torch to cut it, and the enamel will melt.

I've cut holes in similar tops using a round bimetal hole bit and a bit of oil for lubricant.

For a larger hole, I've used an angle grinder, cutting from the back. Have to be careful to not excessively overheat the metal, so I make the cut with several passes, going deeper each time.

I have taped the top or enamel side along the cut line, but I never noticed a difference in the quality of the edge when taking that extra step.

The key it to not over-flex or overheat the metal.

Mongo

Thanks so much to all! I love coming back with this kind of info when I've been discouraged from trying something out of the ordinary. Seems like too many in this building industry would prefer you just stick to the "standard" and not ask for anything atypical that would require a little extra effort or research. They all know by now to expect me to say, "I read on the gardenweb..."

Thanks again for all the advice, and warnings. I can always replace the top with something else if it doesn't work.