In B.C. we do have an agricultural land resource that is there to protect the production base. In theory, it could work. In practice, there has been a lot of erosion, especially in some of the areas of high population density. If this were a really strict reserve with absolutely no opportunity for exclusion, it would probably work.
Ontario is trying this with the greenbelts, as well. I think it is improving in British Columbia. I think there is more of a will to actually keep the reserves.
The other option is for government to buy up all agricultural land that comes for sale. The question is, does government have the money to do this? The answer is probably not. But that would be one option, that government could actually set up a land trust and then lease it out to qualifying producers. There are precedents around the world for this as well.