Peeps, just a few points. And, these are just what came into my head after reading all the above, they are not in any way meant to diss peoples ideas or opinions ok
1) Mini would not suit a 4x4 powertrain. It would be very difficult to incorporate 4wd into the existing engine/gearbox and short of using a morotbike engine in the front longitudinally mounted, no suitable powertrain would.
2) Mini was created as front wheel drive and unless BMW have engineered a new floor plan for around 2008, i cannot see BMW making a suitable business case for re-tooling a new floor pan. I am certain the demand would be low relative to the number of total Mini sales. 5% maximum 4wd sales would not cover investment costs. No business case.
3) Little benefit in dry conditions. 4wd would only eliminate power-on understeer and maybe buy you 0.5 seconds off the line. Is it worth spending more than the value of a new mini on a conversion that does this? Ok, wet conditions would be a benefit such as pulling out of a T-junction, but, in the dry, ultimate mechanical grip provided by the tyres on a 2wd mini would be no greater than that of a 4wd mini.
4) Modern Evo's and WRX etc. All the road going rally cars i have seen on the limit tend to inherently understeer anyway. Car manufactureres are engineering this into the handling of the car specifically for the 'front wheel drive generation'. Why you ask? Think about it.
5) Performance benefit? Hmmm, increase vehicle mass, double amount of rotating drivetrain, increase friction, lose driveability, lose fuel economy, naaa, it just aint stacking up. If ya gonna be an ice racer or rally-cross racer then its all good, but for driving on ordinary roads, i wouldn't bother.
At the end of the day it is up to the person who has the $$$ and is prepared to undertake such an engineering task. I may appear to diss the idea, but wait, i am an engineer and would amire such a conversion and it owner for taking on the task.
Dont forget peeps, owning a mini is all about being individual, and a 4wd certainly would be that.