Although I haven't done it I know the area reasonably well. Ending up unceremoniously dumped on the beach halfway between Lancelin and Wedge with a minor injury or gear failure is probably the most likely scenario if something does go wrong.
But what are your options?
Walk out? 12-13km along the beach carrying a kite, wetsuit, harness, bar lines, board, water etc etc is going to take the better part of 4-8 hours, not an hour and a half. Assuming you are where you think you are and don't choose the longer option.
If you think going inland and walking out is an option, don't even go there, don't ever consider it an option,
ever. It's probably an average 10-15 km to Indian Ocean Drive along there. If you get truly stuck on the beach stay there, it's easier to search a beach 20 m wide for 30 km than it is to search
250 sq km of coastal scrub. Plus you might stumble into the Impact Area and get blown up (there is a good reason why the clearance divers are the only ones allowed in that area).
Flares, who is going to see them during the day? The defence training area set off rocket flares all the time and they largely get ignored.
EPIRB, hardly a life threatening emergency and not appropriate. I'd take an educated guess that an EPIRB activation up there will likely end up with a sea rescue vessel from Lancelin and/or Cervantes/Jurien being deployed, along with the RAC Rescue Helicopter and maybe even the Dornier from Perth Airport. Even then it's not an immediate response, you wouldn't see anyone for an hour or two and they will be pissed at getting deployed due to lack of planning on the part of the kiters. Don't confuse a rescue service with a recovery service. You will have either wasted volunteers time or pull rescue services away from a real emergency, or both.
Weta has indicated he has a 4WD and has driven the beach before, so you potentially have it covered. But then you have to consider how much beach is left after the recent storms, how soft is it? can he get through? How will he know where to pick you up from, are they using the bombing range this weekend? if they are will the sentries stop him from driving up the beach to get you?
The
'she'll be right' attitude will only get you so far. I've seen the boys set off from Lancelin many times, and also seen them passing by whilst kiting at South Rocks and think 'good on 'em, looks like fun' but I also wonder what they would have done if something went pear-shaped.
To help you out in your planning, you should get Telstra coverage all along there, at the very least on top of the nearest sand dune on the beach. If someone does end up on the beach and you are not sure how far north of Lancelin you are look for a vehicle track off/onto the beach and those little 4WD recovery signs with the phone number on it. Those phone numbers could come handy, secondly look for another number in the top corner of the sign, that number will indicate how far north in kilometres of Lancelin you are if you drove via the old Wedge track with the zero point being the gravel road into the off-road area at the top end of Lancelin.
Some of us may sound all doom and gloom, and you hardly have to plan a military style operation, but a little bit of forethought and planning now is going to help a lot if something goes wrong.
Hell your planning need only be to all swap phone numbers, everyone carry water and snacks, all have phones wrapped up and waterproof, and a couple of EPIRBS between you. Then have access to one person with a decent 4WD willing and able to pick you up somewhere on the beach if needed. If you do get stuck on the beach you stay there and wait on the understanding you could be waiting for several hours and it will cost at least a carton for Weta to come get you

I'll try and find out whether the bombing range is in use on the weekend and let you know, I'll also get the caretaker for the range's phone number as most of that coast (including the beach) is defence land it might be handy to have as well. Keep in mind they only permit access along the beach when the range is not in use, access inland off the beach isn't allowed.