Is there a write up on this conversion?

Good call, I've been wondering about this too!

I did this a while ago. Quite straightforward. I made up brackets to bolt onto the original Mazda runners. Legs are required for the rear of the runners. As I usually have the kids with me, we currently only have the drivers side fitted - it's harder to access the back with the MX5 seats, and you loose 2-3 inches of legroom. I'll upload some pics. Happy to answer other questions.
Mine could do with a reupholster, but the best bet would be the pull on "covers" that they flog on ebay - though these cost more than I paid for the seats!

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'68 Deluxe w/Metro engine

See attached pics.
I used the screw in ends from bicycle handgrips as plugs for the bottom of the rear upright - and have plenty of that tube left if anyone wants some - had to be just the right size to be a good fit on the runner.
The welds have to be good too - there can be a lot of force applied through the seat on the twisty bits!

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'68 Deluxe w/Metro engine

I was looking at other forums the other day, sounds like NA seats are the best because NB seats sit up a too bit high?

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did I tell you that I won a trophy?

I'm not certain on that. I have an NB (complete car), and I think the seats fitted to my mini are the same.
One thing is for sure, mine have speaker slots in the headrest, which is a very cool option.
I am not sure which Mx5 seats do or don't have these slots - but I do know that not all of them do.

Cheers

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'68 Deluxe w/Metro engine

My '92 NA has the speaker allocation in the seats. I can't remember where, but I thought I've read the NA seats are lower than the NB... not sure though.

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1970 Mk2 Cooper S

I must admit that as much as you consider this is all good I would suggest getting an automotive engineer to check this out. To me the alterations that you have carried out do not look sufficient for the task and this would only show up in high stress situation.

This is just my opinion.

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"Investing my Super in a Cooper" &
"Re-living my Surfing Past in a Woody"

One thing I did when fitting a set of NA seats years ago was swapping driver's seat to the left and passenger's seat on the right so that the adjustment and tilt knobs are in the middle of the car and don't foul on the door pockets.

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Too many cars, and too little time.

I appreciate the concern Aussie Brian, but I take the view that this is no less safe than the original mini seat mounting. In any crash I imagine the seats in a mini pull out / break off quite easily from their 1/4" thumbscrews.

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'68 Deluxe w/Metro engine

I'm with Brian but can imagine an epic argument, there's no way my engineer would pass those, and they should be engineered, modern seats all seem to use like M12(can't remember) or more bolts I imagine it's for a reason.
My mountings created a new cross member over the front original crossmember and another new one for the rear of the seats, originally I was going to use a 2nd mini cross member but it held the seat too high the seat angle means I needed the front one too unlike AWDmoke I offset my seats towards the middle to clear the door pockets the bonus is the steering wheel is dead ahead, the downside (sometimes... ) is you might touch your passenger's leg. mr2 seats btw, lovely!
Engineering and car safety has improved a lot since the mini was invented we have things like seatbelts! I'm not sure it's much reassurance how the mini seats would have performed.

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Roads need more corners
A Deluxe(CG13DE), 2 Clubbies(998 and 1275) and 2 Morris 1100Ss

Problem is that if the original ones snap off it would be the manufacturers fault and they'd say it met rules at the time of manufacture. If you fit them and they fail at your mounting and someone gets hurt, YOU'RE responsible regardless if theyre stronger than original or not.
That's why engineers want stuff rediculously over engineered. I wanted to swap two seats over in my bus, the originals held in by four 8mm bolts into piddly captivated nuts. To fit new seats he wanted 10mm bolts through floor, 75 x 75 x 3 plates x 4, welded into position. Needless to say, it still has original seats.