I see the risk. However, after 4 years at the company, I felt I should have been trusted. Here's the rest of this particular story:

A year later, I said screw it and worked on it anyway, for 2 weeks (manager was out of town). It was an internal tool. It became the most popular internal tool in the office (95% of people have it open in a browser tab all day). At one point, the founder checked it out and said "I love [name of tool]!". Vindication. I left the company after that, and they use the tool to this day.