Hi Will & other Makers!
This is my 1st post here so please let me know if you want to have a chat in more detail / If I missed anything!
I volunteered to help construct some small stackable dining tables for the Platform community cafe (opposite Loughborough Farm in Loughborough Junction) with a similar kind of finish on the tops, we didn’t use gas burners just a small amount of paper and some scaffolding boards held together in a vertical prism with wire. The boards were kept apart with some wooden wedges - (important to allow the burn to reach the very edges of the boards). I’m not sure how this would work on ply; with the scaffold boards we had a lot of scope for wire brushing the charred parts without having to worry about going through the thin veneered layer. I guess you could experiment with this and try burning it for less time? I have a few pictures of the process and finished tables but can only put one up as a “new user”. You can see the finished tables “live” if you pop in to the cafe, it’s just down the road / 1 stop on the train!
It sounds like you need to find a location to burn but when you get that sorted I’d recommend having some fireproof gloves and a decent sized watering can to hand (ieadlly full of water!), when you (literally) knock it over to put it out you’ll need to give it a good soaking.
Wire brushing the boards back was a really filthy job, definitely wear a mask for that and the sanding too.
Once finished we used a natural Osmo oil to seal the boards, you’ll probably need 2-3 thinly applied coats of this.
Hope that’s in some way useful to you,