There are some viable options for lights.. but I would guess you would want a 12v high difusion type bulb.. they would also have to be mounted up on the pole fairly close to the flag to be effective and last thru the night. Just a quick check of ball park prices puts a 12v solar cell at 5w output at about 80 bucks. Figure in a storage battery, photocel for control, etc, and the lights themselves, plus a container/shelter for the battery, etc.
On the plus side... free power for the lights.. though, may never pay back the initial investment.
If you do run power.. save power.. don't light from the ground.. get smaller more efficient lights like I mentioned, and light from a crosspole under the flag.
Space it so that the flag can hang full length without contacting the lights, etc.
Soundguy