The effect that you are describing is that of adding a nucleation point (or site) to the kettle, as you said. A nucleation point provides a place for bubbles to form and release and will help to prevent boilovers. That being said, though, a penny (or a few pennies) will not have enough surface area to help a batch that large. At places that sell cooking gadgets, you can get a ceramic "boilover preventer". I have used these for cooking, and they work pretty well.
As for brewing, though, I have to recommend Fermcap-S. It is one of those things that once you use it, you will never know how you brewed without it. The cost is very minimal, when you determine how long a bottle lasts you. Last night's brew session, I realized I didn't have any and started without. I was having really bad foaming issues that was causing me to back off the boil, so I added a couple of Gas-X pills. In a pinch, this is something I have done a couple of times, and it works well - although not as good as Fermcap-S. Cost per batch will easily fall into favor of Fermcap-S, as well, when comparing the two.