The possibilities are endless... In general, there aren't a ton of people camping on the beaches in that area around that time of year. The majority that are, will most likely be in the more "established" areas & camps. The few days right around New Years Eve can be a bit crazy with a ton of folks shooting off fireworks in those areas. It's pretty safe to say you can get down there during the day & look around for a suitable spot to camp for the night, or several nights. Here's a few things to keep in mind:
Most of the dirt roads leading to the beach from Hwy 5 do not have gates, so you can drive in and take a look. Some of the land will be open & some will be privately owned. Even if it's privately owned, there may be no one around & no one on-site. If there is, they may allow you to camp on the beach for a night or more. The only way to know for sure is to ask. You may find a spot that looks deserted/open and set up camp, only to find that someone will come around and try to charge you for camping there. This may be legit & it might not. You might not be able to tell.... They typically charge $10 to $20 a night & you can sometimes negotiate. If there are closed or locked gates, or signs staying to stay out or no camping, your better off finding another spot. Some of the "Campos" that have homes in them also have a few campsites or a designated (if you can call it that...) camping areas. Most will have only a cabana or palapa with nothing else. If you are going to be by yourself and boondock, I'd avoid any spot where you can be seen from the highway.
If you haven't been to this area, the tides can be quite extreme, especially around the new & full moons. It's easy to find "the perfect spot" on the beach to set up camp, only to find out that the high tide line is above your site. Take a close look around before setting up camp & make sure your in a safe spot. Also, be VERY wary of the mud flats (or any mud for that matter). They can look dry on the surface, only to be wet/soft just an inch under. Over the years, we've seen a number of folks get their vehicles stuck in the mud and not be able to get out before the high tide comes in and floods their vehicle.
Something else to be aware of with the tides; pay close attention to the road going in to the beach. There are several areas where you can get out to a nice secluded beach and find a good spot to camp, but the road getting in drops below the level of the high tide. If you're not careful, you can get yourself stranded. At least until the tide/water level recedes enough for you to get back out. There are a number of these places that have multiple ways of getting in & out, so just take a good look around and consider these possibilities before setting up camp.
This is the time of year when the cold north winds can blow for days at a time. I'd suggest looking for a spot where you are a bit protected to the north. Also, be sure to secure everything before going to bed at night. The winds can go from 0 to 30 mph in a matter of minutes. Please believe me when I say it's NO FUN chasing your camp chairs down the beach in the middle of the night, in your chonies, in a cold north wind!!!
All that being said, we've had some awesome weather and some phenomenal trips in that area during that time of year.
Hope you guys have a great trip...