I agree with most of what you say. Our school offers home-made 'courses' in short-strip flying, grass field ops, converting from US flying to the UK, dead reckoning only nav, and long-distance route training. Obviously they're not recognised courses but people quite like them even though they don't get a shiny certificate at the end of it. I think the problem is a lack of imagination in developing these courses largely because GA training, microlighting and PFA people don't mix all that much in the sense of developing business at a club level.
On the other hand I don't favour the idea of developing formal courses as I suspect this would make it to easy for authorities to prevent access to farm strips, etc. without the appropriate ratings. It could even lead to having a tarmac and grass rating, for example.
Why training is limited to licensed airfields is a mystery to me. The arguments that exist simply don't seem compelling enough and I'm sure tinstaafl's tale is the right model to follow.
My preference would be for instructors to have access to farm strips, etc. (licensed or not) as part of the PPL training or simply as a 'safety pilot' after training. I'm actually in discussion with a nearby farm strip owner about this possibility for our school (obviously only for those who already have their PPL!)
Regarding the point about using non-CPL instructors, there's a thread in the instructor's forum on this and it's a contentious one! My own view is that it shouldn't be permitted.