Michael,
So these providers are providing the free transit to their non-customers?
This does not make any business sense; it will not happen.
The proper solution is for all these companies to form a consortium. The consortium would run the NAP and contract with multiple NSP's for service. In that case, the NSP's are not providing transit to non-customers because the consortium is the customer and every ISP who joins the consortium gets multihoming reliability outside the region.
Forming a consortium is certainly an alternative for ISPs that aren't big enough, so that they don't provide a degree of addressing information aggregation sufficient to justify their routes to float throughout the "default-free" zone of the Internet. The consortium would acquire a (faily large) block of addresses, which would be partitioned among the members of the consortium. The consortium could run its own "backbone" (e.g., NAP) that would provide all members of the consortium with connectivity to other parts of the Internet. The basic connectivity (what I would call "route push") would be provided via this "backbone". The "backbone" would also perform address information aggregation into a single prefix. Additional connectivity can be acquired by the individual members of the consortium via "route pull", but this connectivity would be just to get better routers. This way a small ISP would have a choice of joining a consortium (and there may be a choice of consortiums to join) and get its addresses out of the consortium's block, or to connect to a large NSP and get its addresses out of the large NSP. However, one needs to understand the consortium model has its own issues ... Yakov.