Palacerigg are hoping to convert one of the paddocks beside the drive into temporary parking for big events, which should ease the problem.
Ingliston have been approached and are not really interested. Also, it is an extremely expensive venue, so the only possibility there would be to combine with another event such as the Gardening thing. Thinking about how much it costs to enter animals for the Highland Show, there wouldn't be many entries at that site for a smallholders event.
Kirkintilloch would be very interesting although it would remain smallscale I would think. But it could set the trend for other local agricultural shows to go down that route.
For Lanark, there is a large field across the road from the mart where the Medieval Fair is held (the racecourse

) , which could combine with the mart itself to host a large smallholders event, with room for lots of stalls and so on. Lanark does have the advantage of being accessible from England and from the southern half of Scotland, but the Central Belt might get overall greater numbers.