A fascinating thread so far. I'm an N scale modeller from Australia, doing a lot in 1:160, building locos from scratch as well as having an interest in G gauge electric in the garden. Just got a book on building your own live steam locomotive (it's from 1976 and recommends using Asbestos as lagging!) from my grandfather in law and have been seriously thinking about having a go at 5 inch gauge by joining the local club as a track at home is not realistic to say the least.
Model engineering as others have already stated, I agree extends to what most of us on RMweb do in the "smaller scales". The thing that frightens me is that it could take a fair while to complete a live steam locomotive, or least that's the impression I get, compared to building in N scale. I suppose though it comes back to multiplying the scale up, it would have to be more like 16 times compared with 8 for OO as already mentioned.
I have considered doing live steam first in G gauge and then working up to 5 inch gauge once I get the basics of building a steam engine that actually employs the old order and not just an electric motor driving a gearbox with some rods going around for the ride.