I fully agree with Eaglecliff - there is a fundamental problem with having shows and displays that showcase a hobby that is practised principally outdoors in the garden, for full effect, that are locked in a stripped down indoor environment. Yes, there are some beautifully made indoor layouts on the "circuit" but they can never convey the full possibilities of what can be achieved in a garden setting. The problem is those fully realised outdoor larger scale railways took about 10 years to mature to their full majesty, not exactly what is required for a one day annual show. Not the case with smaller scales where an intricate, compact and portable layout can convey the participants' skills and imagination to the full. Anybody remember the "County Gate" 009 extravaganza.
The only exception I can think of are the fairly bare G1 "race tracks that are high on finely detailed locos and stock, but that is their natural environment. They draw admiring crowds of non adherents wherever shown through the drama and sound they issue. A bucolic vision of narrow gauge idyl was never intended to achieve that.
The shows we attend, irrespective of scales catered for, largely preach to the converted. I doubt they actually bring in many new practitioners to our hobby, except perhaps to make those already heading our way are aware of the range of product available to run on their chosen track gauge. If the trader element has failed, at this end of the hobby particularly, then you are heading for oblivion - you no longer have a reason to be, so to speak.
Sorry to be so blunt.
The garden centre solution - could be quite novel. I'm sure most have a "dead spot" in their trading season where a garden rail show could be seen as a way to drum up a bit of extra footfall. They have indoor bits to house electrical equipment and a fair old bit of stock that could be deployed a la "Garden Rescue" to create the right effects. Max