I would make it officially OK and inform my potential employer of the intent to do work outside of your agreement with them. Above board is always better. A word of advice though once you have an official OK in writing, don't talk about your other work at your job. You can read the famous story of Woz, the Apple 1 and HP for the reasoning why you should not. In short they said they where not interested, he built it and then they wanted a slice of the pie. It's better to get it in writing before any side projects and then never mention it again.