Although I've only gotten 1 girl from each of the last 3 events (and I used to get both), I think the 1 in 300 chance is a little low. (although I can easily believe the regular girl drop rate is less than 1 in 5000).
I usually am able to fight 3 times a day during events, so thats 15 attempts a day. in a 14 day event, thats about 210 attempts. I usually find the first girl about day 8.. at have always found at least 1 girl so far. So my guess is event girls are closer to 1 in 150 or 1 in 200, than 1 in 300.