I got an email back from that chap at hyper-formance.com
I asked him if they ever did a version for BMX's but he said they never did because they had no sissy bars to mount it to.
So its either having a big long seat with sissy bars (if they fit an 80's BMX) or having to engineer something to take the load.
Some old skateboard parts and some odd bits of metal.
I'm no good as an engineer so its up to you.
Surely its worth trying to build it for the novelty value.
Plus the laugh rating will be high.
Hopefully someone will turn up to MK08 with a device.
I'd really like to see it and make sure you video it.