It's probably wise to consult a lawyer about this, but for my part, I say go ahead and work on your employer's time. Understand that you are taking a calculated risk. If the risk (e.g. getting fired, losing ownership of your work) isn't worth it, don't do it. But if it is, then stay as productive as you were before at work while trying to be as efficient as possible (e.g. by using methods in the Four Hour Work Week) to squeeze out extra time for your project. On your own time, work as hard as you can without skimping on your other obligations (e.g. family, bills, etc.).