I have retro fitted door bars to a few minis in the past, they were just a copy of rovers efforts but with two bars on each side.
They can be fitted without removing the door skin, just the winder and glass. Oh and you need fabricating skills :)
The other good advice I was recently given by a paramedic admiring my mk3 was to get some seats with headrests as he had the unfortunate experience of attending an RTA where the driver was hit hard from behind and suffered fatal and catastrophic head and neck injuries .
I`m far too old for boy racer seats now but 30 odd years ago I ran out of talent and rolled a mini at speed. It was like a pin ball machine in there !
Anyway crawled out of the car unscathed and every thing was folded around the quality bucket seat that had acted like a roll cage.
I followed a car yesterday full of very lost people on holiday doing a big of sudden lane swapping and just generally being in the way. there was a sticker on the rear window saying Smile you`re on CCTV. I gave them plenty of room and didn`t go hurtling past like I normally would in a hurry to get home from work on my motorcycle.
I looked for but did not see a camera so i have to get some sticker to keep the idiots off my tail when i`m sticking to the speed limits in my minis, good safety addition a reckon.