This is an interesting idea. As I see it there are a few ways that it can be made more financially feasible while staying true to the ideal of preserving the sim(s) as a natural wilderness area:
1) Offer a portion of plots for sale anyway but create a covenant that only allows for 'gardening' or 'natural wildlife' style decoration - i.e. no furniture, pavillions or houses - and forbids using whitelist access control. I think we would be surprised how the concept of a sim without houses could appeal nevertheless to budding landscape designers, who have nowhere else to go in SL if they want to buy land that is guaranteed to resemble natural wildlife surroundings. If necessary, a subportion of these parcels could be permitted to have skyboxes above 500 metres or whatever is necessary to ensure they stay out of view from any of our sims.
2) Go void. Put void sims all around Neufreistadt as suggested by Salzie. Sell (portions) of the low-primmed land anyway in a 'sponsor' fashion under the same restrictive covenants and maybe one or two parcels in each void sim for 'real purposes although we'd need to look into if the void sims lower ability to support many avatars could pose a problem with regard to hosting public events. In any case if we went the void way we would need to discuss whether the low-primmed land would give rights of citizenship.