Are there Dynamo powered, rear flashing lights?
I have an Origin rear light and I've seen that it can be charged via usb and also have the lights on  at the same time. If you want to have it powered by a Dynamo, what you could do is have the Dynamo connected to it the whole time on the bike and be lighting at the same time. That could be a way to pull it off and never have to take it off. Also, if you have daylight, the lights could be off and charging so you can use it when the night comes. Good luck.