Could evidence of a signal from an exocivilization be found in the geologic record?
No, it is not possible.

The only thing I could think about is re/de/magnetisation of magnetic minerals. Even then I'm not sure it's feasible, because you to form a magnetic field you need an electric current in motion, but that's a question for physics, not here.

So far, the major magnetic signals recorded in the geological history are well understood. There are many small scale signals that are less well understood, and are believed to be the result of internal processes within the Earth. You can look at my answer here for examples of these signals. Anyway, even if an extraterrestrial civilisation could somehow affect the geological record, it would have to be sustained for very long times (hundreds to thousands of years) for us to be able to see something, and even then we probably wouldn't be able to distinguish it from noise.

As an example, think of a river with rocks in it, constantly getting moved and tumbled. Then you go and throw a rock inside. There would be absolutely no way to know which rock was it (and even that someone threw a rock inside), even a day later, let alone 100 million years. The only way to distinguish something would be a huge amount of rocks, not just one. And then there's the question, of why would someone spend this enormous amount of energy of throwing rocks in the river or sending EM beams towards planets?