Given that it would need so much more than just the basic power supply in electrical terms, and a large variety of parts mounted in difficult places to access, I'd imagine a climate retrofit would be a whole order of magnitude harder than an OSS retrofit. While the latter does involve relocating one of the roof's lateral structural supports, it's otherwise a mainly physical challenge to fit a complete component assembly. And to date I've only heard of one OSS retrofit (actually seen it too, on
@Phoenix's 1.2TDI at AITP a couple of years ago), although I'm sure there must be others. I'd say a climate retrofit is therefore as good as impossible for most intents and purposes.
Just to note for safety's sake, for those who haven't seen the subject discussed before, while the 4-5 seater conversion (or vice versa) looks like it'd be easy, the difference between them lies in the design and strengthening of the entire floorpan of the car. A definite no-no to attempt ?, don't go there.