I appreciate this is a rather basic solution...but would it not be possible to have a kind of hayride for those visitors in wheelchairs? When I was little and visited farms in California...they would often have a hay trailer hooked up to a small 4x4 or tractor that would pull us around the farm. If the trailer was made wheelchair accessible then the visitors could still enjoy the sights and smells without having to struggle with the terrain. I think the only problem with virtual tours is that the visitor does not have the best possible experience of the farm. Many disabled people appreciate feeling the wind on their face and the different smells of new envirnoment or the sounds od somewhere new because their other senses may have been weakened through their disability. A simple hayride would bring all of these whilst still being inkeeping with the traditional feel of the farm.