Here is an answer: I just bought the Cygolite Hotshot Pro 150. I took it out of the box, played with it a while and then put it on a "5v 1A" (5 Watt) USB power source to charge. I also turned it on to the highest steady mode. Five and a half hours later it hadn't heated up much, still was shining brightly, and still wasn't fully charged. According to the box it came in, a full charge should have only powered it in that mode for two hours. The box also indicated that normal charge time is five hours. Without talking to the engineer who designed it, there are a number of unknowns, some of which are potentially dangerous. (Did I mention that I did this test INSIDE a cold oven for safety?) It isn't clear if it was charging at all, or just powering the light directly from the USB power port. (This seems unlikely.) Assuming that the unit was still charging slowly, there would come a time when the internal battery would be fully charged - but would the charging system detect that and shut down, or be confused by the current drain of the light - perhaps dangerously so? At the very least I would expect the usable battery life to be decreased. Following my experiment, I will purchase a USB battery bank to use with it on long bicycle tours, but I will only run and charge simultaneously in emergencies - not as a regular thing. Experiment at your own risk.
Read these quotes from batteryuniversity.com: "A portable device should be turned off during charge. This allows the battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low enough by drawing a leakage current. A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress."
"Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V on a Li-ion designed for 4.20V/cell will plate metallic lithium on the anode. The cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises and if the charge is allowed to continue, the current interrupt device (CID) responsible for cell safety disconnects at 1,000–1,380kPa (145–200psi). Should the pressure rise further, the safety membrane on some Li-ion bursts open at about 3,450kPa (500psi) and the cell might eventually vent with flame."