Hmm good point about the steering lock. That's technically an MoT requirement, although I don't recall ever having seen it tested! I think adapting a more modern electronic one would be quite a challenge. Mechanical locks often require some tension releasing from the column before you can turn the key, whereas electronic ones can't do this so they must be designed differently from the start to ensure there is never any mechanical force which can prevent them disabling.
Given that the usual method of stealing cars like this is to break in to the corresponding house and take the keys, I'd be less concerned about adapting the factory security stuff for some added convenience
I like Ainars' solution with a remotely positioned transponder. You'd still have to defeat the aftermarket security in that case, and if its a decent system which is well installed (ie carefully wired in out of sight rather than a cluster**** of scotchlocks and sticky tape under the steering column like most aftermarket fitters used to do) then that's no more or less easy than defeating the factory stuff.
The brake pedal logic would work well.
The keyless go in the D3 relies on the security modules giving the ECU authority to start the engine - its quite a complex digital sequence to make it do this so unless you transpose most of a D3 in to a D2 I don't think adapting something from another model is feasible in this case. Its likely to only be possible using mostly aftermarket stuff.