I have a set of pipes in my bathroom that are subject to freezing in the wintertime due to a home addition which allows air to access space in the walls. We tried adding insulation to the outside wall where the air comes in but it is not enough. The pipes are accessible through a drop panel and there is an electrical outlet nearby. Are there small space heaters that are used for enclosed spaces? Or pipe heaters? I'm worried about them getting wet as the space is underneath a tub.

I have had a problem also with pipes freezing and then bursting in the winter. There were two things that I did in order to help the situation. One, was that I created an extra furnace duct and ran it up into the wall where the pipes were freezing. Two, I cut a hole in the wall by the pipes that would freeze and put a vent in the opening that I had created so that warm are would go into the vent.

There are electrical cords that are made for heating pipes that you wrap around them.

electric pipe insulation - Google Search

There might be someone with a better answer for your question, though.