The Davis, CA randonneurs typically start their 600ks on Friday night at 8PM. The San Francisco Randonneurs usually have a 200k that starts at 8PM, so, yes, it does happen. One common rationale for doing them this way is to help PBP-bound riders get experience riding through an entire night.

I haven't done a long ride starting at night, but it seems like your reasons for doing so make sense. The one thought i have though: If you're trying to minimize the total hours ridden in darkness and hours where services are likely to be closed, does it make sense to start even later (provided you have that flexibility)? In other words, if you're finishing at 5PM, you're still leaving a couple hours of daylight on the table.

It might be that waiting to start until 11PM or midnight could make you feel sleepier than if you start at 9PM -- at 9PM, you're not really thinking of sleep yet and by the time 2AM rolls around, your heart rate's up, you're well into the ride, it may seem easier to make your way through until dawn. So maybe 9PM is the best choice.

But if you're pretty confident you can stay alert through the night, and your primary goal is to minimize total hours of darkness/no services, then an even later start may be the way to go.