not many beaches your allowed to drive/camp on down there but I think Boranup is ok, good surf too

every beach Cape to Cape is national park so other than Boranup and Deepdene there is no beach access for 4x4.

Also pretty sure that beach camping is not prohibited not that it stops some people. Having said that I wouldn't push my luck over the xmas period. Rangers and DEC will be out in force pinging people doing the wrong thing throughout the region.

Dont know if they have opened up Kilcarnup(north side of the Margaret River Mouth) since the fires last year but people always used to camp there and along the beach to the north nearly all the way to Ellenbrook along the coastal/beach tracks But they arent actually supposed to......apparently:P. Bit of fun beach driving too as the beaches are usually hard. 

Kilcarnup itself is/was? actually on council land so administered by Margaret River Rangers but to the north past it it becomes National Park so administered by DEC. In my exeperience down there generally the council Rangers were quick to get their notepad out, but the DEC Rangers i found didnt mind as long as you werent trashing the place, had pets or lighting a camp fire(kind of ironic for down there though).

Some others i have camped at, usually only overnighters for fishing so usually get there late arvo and gone pretty early in the morn. Before the Rangers start work :D

Wilyabrup beach. I have done some overnighters with my 4wd there. Good fishing at times :)

Out the back of Indjidup Point(Cape Clairault) to the south

South of Redgate and North of Contos the area around Bobs Hollow is nice for a night.

As stated Boranup beach or the north end of Hamelin Beach.

South of Hamelin around to Elephant Rock or Deepdene.

Note that most of these places are in the National Park so you arent 'supposed' to camp outside of designated camp areas. Think Contos and Point Rd are the only DEC approved ones?

Great book to get ahold of is the Cape to Cape Track guide book. All the maps in there actually also show all the 4wd tracks. In actual fact alot of the Cape to Cape track IS on 4wd tracks, though some have now been gated shut.

Hope this helps. Oh and if you want to get brownie points if a Ranger does pull you up take a few Garbage bags and do a bit of a clean up of any rubbish you can find ;)